staging: mt7621-dma: Remove assignment in if

Fixes checkpatch error: ASSIGN_IN_IF by adding an inner if in the else
path, this also avoids calling vchan_find_desc when not needed.

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