spi: spi-mem: Extend spi_mem_adjust_op_size() to honor max xfer size

Extend spi_mem_adjust_op_size() to take spi->max_write_size and
spi->max_read_size into account.

Signed-off-by: Vignesh R <vigneshr@ti.com>
Tested-by: Simon Goldschmidt <simon.k.r.goldschmidt@gmail.com>
Tested-by: Stefan Roese <sr@denx.de>
Tested-by: Horatiu Vultur <horatiu.vultur@microchip.com>
Reviewed-by: Jagan Teki <jagan@openedev.com>
Tested-by: Jagan Teki <jagan@amarulasolutions.com> #zynq-microzed