V4L/DVB: gspca - main: Fix a crash of some webcams on ARM arch

When plugging some webcams on ARM, the system crashes.
This is because we alloc buffer for an urb through usb_buffer_alloc,
the alloced buffer is already in DMA coherent region, so we should
set the flag of this urb to URB_NO_TRANSFER_DMA_MAP, otherwise when
we submit this urb, the hcd core will handle this address as an
non-DMA address and call dma_map_single/sg to map it. On arm
architecture, dma_map_single a DMA coherent address will be catched
by a BUG_ON().

Signed-off-by: Jason Wang <jason77.wang@gmail.com>
Signed-off-by: Jean-François Moine <moinejf@free.fr>
Cc: stable@kernel.org
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>