Bluetooth: drivers: h4: Fix uart_fifo_read return value handling

Make sure negative error returns from uart_fifo_read() are correctly
handled.

In the same go, the logic of reading packet headers (ACL/event/ISO) is
refactored into its own helper function. This also fixes having an
appropriate name for the variable that tracks how many header bytes have
already been read (it was called "to_read" and now it's called
"bytes_read").

Fixes #39805

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>