spi: fsl_espi: fix read transactions

spi: fsl_espi: fix read transactions

Since commit 7917c2e35604 ("spi: fsl_espi: fix din offset") MTD is
basically broken because any read transaction will get wrong data. While
the commit in question will fix simple transfers (where both
SPI_XFER_BEGIN and SPI_XFER_END is set), it will break the most common
case, where opcode and address is send first and then data comes as a
second transfer.

This basically reverts commit 7917c2e35604 ("spi: fsl_espi: fix din
offset") and make the fix particular for this simple case. Instead of
providing two buffers for reading and writing, just malloc one which is
used for both. This will work because the data is first written on the
SPI bus and then it will be read (and overwite the written data) into
the same buffer.

Suggested-by: Tomas Alvarez Vanoli <tomas.alvarez-vanoli@hitachienergy.com>
Fixes: 7917c2e35604 ("spi: fsl_espi: fix din offset")
Signed-off-by: Michael Walle <mwalle@kernel.org>
Signed-off-by: Peng Fan <peng.fan@nxp.com>