Some combinations are simply not valid and should be rejected before
the op is passed to the SPI controller driver.

Add an spi_mem_check_op() helper and use it in spi_mem_exec_op() and
spi_mem_supports_op() to make sure the spi-mem operation is valid.
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Signed-off-by: Mark Brown <broonie@kernel.org>