[media] radio-usb-si4713: make si4713_register_i2c_adapter static

This function isn't used nowhere outside the same .c file.
Fixes this warning:

drivers/media/radio/si4713/radio-usb-si4713.c:418:5: warning: no previous prototype for 'si4713_register_i2c_adapter' [-Wmissing-prototypes]
 int si4713_register_i2c_adapter(struct si4713_usb_device *radio)
     ^

Cc: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>