[roll third_party/pigweed] pw_spi: Correct full-duplex behavior of linux_spi

Prior to this change, LinuxInitiator::WriteRead() would always perform a
compound transfer, sending write_buffer *then* receiving read_buffer,
i.e. a half-duplex write-then-read operation.

This corrects that behavior to match the interface which describes a
full-duplex (simultaneous write and read) operation.

Note that `struct spi_ioc_transfer` only has a single `len` field
describing the number of bytes to send and receive in that transfer.
Because write_buffer and read_buffer could have different sizes,
this implementation will split the transaction into two transfers if
necessary to transfer the longer of the two.

Also add missing "override" to ~LinuxInitiator(), and remove a few
extraneous #includes.

Test: bazel test //pw_spi:linux_spi_test
Test: Execute a test application on a Linux host connected to a SPI
      responder MCU programmed to send a fixed message with no dummy
      cycles.
Test: Call WriteRead() with buffers of the same size. Verify message
      is received in read_buffer. Verify via logic analyzer that a
      single transaction is performed (no extra clock cycles).
Test: Call WriteRead with write_buffer longer than read_buffer.
      Verify (partial) message is received in read_buffer. Verify via
      logic analyzer that two transfers were performed (indicated by
      inter-transfer delay with CS still active), and only
      write_buffer.size() bytes were transmitted.
Test: Call WriteRead with read_buffer longer than write_buffer.
      Verify message is received in read_buffer. Verify via logic
      analyzer that two transfers were performed, and the transmitted
      data was zero-padded out to read_buffer.size().
Original-Bug: b/316067628
Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/192591

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: ff7e7a78ce968db..63903b4f24e9122
Roller-URL: https://ci.chromium.org/b/8755992859306136001
GitWatcher: ignore
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Change-Id: I4724256f488aa8d1c547e07a2bec8fb9e4d80c3a
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/192765
Bot-Commit: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
Commit-Queue: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>