spi/omap-mcspi: Fix the spi task hangs waiting dma_rx

The spi hangs waiting the completion of omap2_mcspi_rx_callback.

Signed-off-by: Jorge A. Ventura <jorge.araujo.ventura@gmail.com>
Signed-off-by: Mark Brown <broonie@linaro.org>
Cc: stable@vger.kernel.org