spi: fsl-cpm: Check length parity before switching to 16 bit mode

Commit fc96ec826bce ("spi: fsl-cpm: Use 16 bit mode for large transfers
with even size") failed to checkout that the size is really even before
switching to 16 bit mode. Until recently the problem went unnoticed
because kernfs uses a pre-allocated bounce buffer of size PAGE_SIZE for
reading eeprom.

But commit 8ad6249c51d0 ("eeprom: at25: convert to spi-mem API")
introduced an additional dynamically allocated bounce buffer whose size
is exactly the size of the transfer, leading to a buffer overrun in
the fsl-cpm driver when that size is odd.

Add the missing length parity verification and remain in 8 bit mode
when the length is not even.

Fixes: fc96ec826bce ("spi: fsl-cpm: Use 16 bit mode for large transfers with even size")
Closes: https://lore.kernel.org/all/638496dd-ec60-4e53-bad7-eb657f67d580@csgroup.eu/
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Link: https://patch.msgid.link/07d1ea39a0e9b56846395d79d489a5153206ff5c.1762714267.git.christophe.leroy@csgroup.eu
Signed-off-by: Mark Brown <broonie@kernel.org>