mxs/spi: Increment the transfer length only if transfer succeeded

The transfer function incremented (struct spi_message)->actual_length
unconditionally, even if the transfer failed. Rectify this by incrementing
this only if transfer succeeded.

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