usb: gadget: f_uac1: Staticize local functions

control_selector_init() is used only in this file.
audio_bind_config() is used only in audio.c file to which
f_uac1.c is included. Thus, these functions are staticized
to fix the following warnings.

drivers/usb/gadget/f_uac1.c:698:12: warning: symbol 'control_selector_init' was not declared. Should it be static?
drivers/usb/gadget/f_uac1.c:722:12: warning: symbol 'audio_bind_config' was not declared. Should it be static?

Acked-by: Jassi Brar <jaswinder.singh@linaro.org>
Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>