usb: gadget: f_loopback: Fix sparse warning

Make local symbols static in order to fix the following sparse
warnings.

drivers/usb/gadget/f_loopback.c:123:34: warning: symbol 'ss_loop_source_comp_desc' was not declared. Should it be static?
drivers/usb/gadget/f_loopback.c:139:34: warning: symbol 'ss_loop_sink_comp_desc' was not declared. Should it be static?

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