spi: spi-mem: s/dummy/data buswidth check in dtr_supports_op()

This should have been op->data.buswidth instead as we check for octal
bus width for the data related ops
Also add explanation for why there is checks for 8D even data bytes

Cc: Pratyush Yadav <pratyush@kernel.org>
Reviewed-by: Pratyush Yadav <ptyadav@amazon.de>
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>