s390: remove unused _swsusp_reset_dma

Since commit 39421627 ("s390: remove broken hibernate / power
management support") _swsusp_reset_dma is unused and could be safely
removed.

Reviewed-by: Sven Schnelle <svens@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>