[media] radio-usb-si4713: fix sparse non static symbol warnings
Fixes the following sparse warnings:
drivers/media/radio/si4713/radio-usb-si4713.c:226:31: warning:
symbol 'start_seq' was not declared. Should it be static?
drivers/media/radio/si4713/radio-usb-si4713.c:291:29: warning:
symbol 'command_table' was not declared. Should it be static?