spi: spi-mem: Compute length only when needed

When adjust_op_size is defined, len is never used. Move the len
computation where it's actually used.

Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Link: https://lore.kernel.org/r/20200228160735.1565047-1-tudor.ambarus@microchip.com


Signed-off-by: Mark Brown <broonie@kernel.org>