spi: spi-mem: Fix read data size issue

When slave drivers don't set the max_read_size, the spi-mem should directly
use data.nbytes and not limit to any size. But current logic will limit to
the max_write_size.

Signed-off-by: Ye Li <ye.li@nxp.com>
Acked-by: Vignesh Raghavendra <vigneshr@ti.com>
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>