spi: mxs: Assign message status after transfer finished

In the current code implementing the MXS SPI driver, every transferred
message had assigned status = 0, which is not correct. Properly assign
status returned from the I/O functions.

Signed-off-by: Marek Vasut <marex@denx.de>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>