media: gspca: fix frame overflow error

When converting gspca to vb2 I missed that fact that the buffer sizes
were rounded up to the next page size. As a result some gspca drivers
(spca561 being one of them) reported frame overflows.

Modify the code to align the buffer sizes to the next page size, just
as the original code did.

Fixes: 1f5965c4 ("media: gspca: convert to vb2")
Tested-off-by: Hans Verkuil <hans.verkuil@cisco.com>

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Reported-by: softwarebugs <softwarebugs@protonmail.com>
Cc: <stable@vger.kernel.org>      # for v4.18 and up
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>