[media] stkwebcam: Fix sparse warning on undeclared symbol

The sparse warning is:
"drivers/media/usb/stkwebcam/stk-webcam.c:59:5:
warning: symbol 'first_init' was not declared. Should it be static?"
Declare variable 'first_init' as static and local to the function.
Found by Hans Verkuil's daily build. Tested by compilation only.

Signed-off-by: Ezequiel Garcia <elezegarcia@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>