spi: imx: return error from dma channel request

On SDMA initialization return exactly the same error, which is
reported by dma_request_slave_channel_reason(), it is a preceding
change to defer SPI DMA initialization, if SDMA module is not yet
available.

Signed-off-by: Vladimir Zapolskiy <vladimir_zapolskiy@mentor.com>
Signed-off-by: Anton Bondarenko <anton.bondarenko.sama@gmail.com>
Signed-off-by: Mark Brown <broonie@kernel.org>