V4L/DVB (13324): gspca - pac7302/pac7311: Handle return value of usb_control_msg().

The function usb_control_msg() can return error any time so at least
warn the user if an error happens. No message is printed in case of
normal operation.

Signed-off-by: Marton Nemeth <nm127@freemail.hu>
Signed-off-by: Jean-Francois Moine <moinejf@free.fr>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>