[media] pvrusb2-io: no need to check if sp is not NULL

The buffer_complete() routine assumes that sp is not NULL,
otherwise it will fail completely. Btw, this is also
assumed at pvr2_buffer_queue(), with is the routine that
setups the URB handling.

So, remove the bogus for the callback at buffer_complete.

This fix this smatch warning:
	drivers/media/usb/pvrusb2/pvrusb2-io.c:476 buffer_complete() warn: variable dereferenced before check 'sp' (see line 472)

Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>