spi: spi-mem: perform odd len check only while writing data

in spi_mem_dtr_supports_op we have a check for allowing only even number
of bytes to be r/w. Odd bytes writing can be a concern while writing
data to a flash for example because 8 DTR mode doesn't support it.
However, reading ODD Bytes even  though may not be physically possible
we can still allow for it because it will not have serious implications
on any critical registers being overwritten since they are just reads.

Cc: Vaishnav Achath <vaishnav.a@ti.com>
Cc: Pratyush Yadav <pratyush@kernel.org>
Cc: Vignesh Raghavendra <vigneshr@ti.com>
Tested-by: Nikhil M Jain <n-jain1@ti.com>
Signed-off-by: Dhruva Gole <d-gole@ti.com>
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>