usbd: hold RX_PENDING in every build, not only with OSAL_MUTEX_REQUIRED

One state bit and one code path regardless of OS/core config, so a class driver
that claims an OUT endpoint from xfer_cb without consuming misbehaves the same
way on bare-metal as under an RTOS, and test_usbd covers it without forcing
TUP_MCU_MULTIPLE_CORE.