mmc: omap_hsmmc: No need to check DMA channel validity at module remove

The driver will not probe without valid DMA channels so no need to check
if they are valid when the module is removed.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
CC: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>