usb: gadget: rndis: Staticize rndis_init()/rndis_exit()

rndis_init() and rndis_exit() are used only in this file.
Fix the following sparse warnings:

drivers/usb/gadget/rndis.c:1145:5: warning: symbol 'rndis_init' was not declared. Should it be static?
drivers/usb/gadget/rndis.c:1179:6: warning: symbol 'rndis_exit' was not declared. Should it be static?

Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>