[media] media: vb2: fix incorrect return value

This patch fixes incorrect return value. Errors should be returned
as negative numbers.

Reported-by: Tomasz Stanislawski <t.stanislaws@samsung.com>
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>