V4L/DVB (12358): gspca - main: Memorize the current alt before setting it

This prevents a loop if a permanent error occurs in usb_set_interface().

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