V4L/DVB: videobuf-dma-sg: Fix a warning due to the usage of min(PAGE_SIZE, arg)

drivers/media/video/videobuf-dma-sg.c: In function ‘videobuf_pages_to_sg’:
drivers/media/video/videobuf-dma-sg.c:119: warning: comparison of distinct pointer types lacks a cast
drivers/media/video/videobuf-dma-sg.c:120: warning: comparison of distinct pointer types lacks a cast

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