The usb_control_msg() can fail any time. Only continue writing
sequence if there was no error with the previous write. If there
was any problem stop sending URBs and propagate the error to the
gspca_main.

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