V4L/DVB (7283): videobuf-dma-sg: Remove unused variable

Removed warning message:

 - videobuf-dma-sg.c: In function 'videobuf_dma_unmap':
 - videobuf-dma-sg.c:281: warning: unused variable 'dev'

Signed-off-by: Douglas Schilling Landgraf <dougsland@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>