spi: spi-mem-nodm: 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.

This commit mirrors the same changes in the dm version done in
commit 535b1fdb ("spi: spi-mem: Fix read data size issue").

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>