staging: mt7621-dma: fix unbalanced braces

Add braces to conditional statements if the other branch has them in
order to be more compliant with the linux kernel coding style.

Remaining checkpatch.pl issues include unnecessary line continuations
and use of non-conventional types.

Signed-off-by: Lou Knauer <lou.knauer@fau.de>
Signed-off-by: Philipp Panzer <philipp.panzer@fau.de>
Reviewed-by: Matthias Brugger <matthias.bgg@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>