[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?

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>