media: usb: gspca: add a missed return-value check for do_command

do_command() may fail. The fix adds the missed return value of
do_command(). If it fails, returns its error code.

Signed-off-by: Kangjie Lu <kjlu@umn.edu>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>