staging: mt7621-mmc: Remove variable num form msdc_dma_setup

The variable num in msdc_dma_setup is only used for a BUG_ON
statement, so it can be removed by inlining the condition.

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