spi: spi-mem: Split spi_mem_exec_op() code

The logic surrounding the ->exec_op() call applies to direct mapping
accessors. Move this code to separate functions to avoid duplicating
code.

Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Reviewed-by: Miquel Raynal <miquel.raynal@bootlin.com>
Signed-off-by: Mark Brown <broonie@kernel.org>