[media] pvrusb2: fix smatch errors

These are false positives, but still easy to fix.

pvrusb2-hdw.c:3676 pvr2_send_request_ex() error: we previously assumed 'write_data' could be null (see line 3648)
pvrusb2-hdw.c:3829 pvr2_send_request_ex() error: we previously assumed 'read_data' could be null (see line 3649)

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>