[media] gspca: Fix a warning for using len before filling it

The check for status errors is now before the check for len. That's
ok. However, the error printk's for the status error prints the URB
length. This generates this error:

drivers/media/video/gspca/gspca.c: In function ‘fill_frame’:
drivers/media/video/gspca/gspca.c:305:9: warning: ‘len’ may be used uninitialized in this function

The fix is as simple as moving the len init to happen before the checks.

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