Revert "drivers: spi: mcux_flexcomm: use rxignore bit instead of dummy read."

This reverts commit a3530d6a.

This change incorrect if chip-select is via a GPIO pin,
released by spi_context_cs_control(), and not the controller
default chip-select pin that can be released using a final
FIFOWR.

When GPIO is used for chip-select control, the chip-select could
be released too soon because the transfer callback is invoked
when the last byte is put into the TX FIFO but that byte might
not yet have clocked out yet on the SPI pins. The rx part of
the transfer is really what completes the transfer so ignoring
it is incorrect.

We could try to special case and use ignore only when
spi_context_cs_control() does nothing, but since it is a very
small optimization, it doesn't seem worth the extra
complexity.

Signed-off-by: Mike J. Chen <mjchen@google.com>