[media] gp8psk: make local symbol gp8psk_fe_ops static

Fixes the following sparse warning:

drivers/media/usb/dvb-usb/gp8psk.c:281:28: warning:
 symbol 'gp8psk_fe_ops' was not declared. Should it be static?

Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>