spi: pxa2xx: handle error of pxa2xx_spi_dma_prepare()

If by some reason pxa2xx_spi_dma_prepare() fails we have not to ignore its
error. In such case we abort the transfer and return the error to upper
level.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
[Jarkko: Avoid leaking TX descriptors in case RX descriptor allocation
fails. Noted by Robert Jarzmik <robert.jarzmik@free.fr>.
Unmap also buffers in case of failure.]
Signed-off-by: Jarkko Nikula <jarkko.nikula@linux.intel.com>
Acked-by: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: Mark Brown <broonie@kernel.org>