V4L/DVB: gspca_main: some input error handling fixes

2 small changes to input device error handling:
1) Make it fatal when we fail to create an input device (it is either this
   or add checks for gspca_dev->input_dev being NULL in a lot of places)
2) Since we allow gspca_input_create_urb() to fail everywhere we call it,
   and thus never check its return value, make it void.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>