staging: mt7621-mmc: Fix bug on dma_alloc_coherent fail

In case of dma_alloc_coherent failing the current code just called
BUG_ON. By adding error handling for that case this can be avoided.
This also fixes a memory leek in case of a fail later on in the probe
function.

Signed-off-by: Christian Lütke-Stetzkamp <christian@lkamp.de>
Reviewed-by: NeilBrown <neil@brown.name>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>