cciss: check for msi in interrupt_not_for_us

Check to see if h->msi[x]_vector is set.  We need this for a following
patch.  Without this check we process one interrupt then stop because in
msi[x] mode the interrupt pending bit is not set.  Not sure why we didn't
encounter this before.

Signed-off-by: Mike Miller <mike.miller@hp.com>
Cc: Stephen M. Cameron <scameron@beardog.cce.hp.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>