usb: gadget: function: f_hid: fix sparse warning

this patch fixes following sparse warning:
f_hid.c:572:30: warning: symbol 'f_hidg_fops' was not declared. Should it be static?

Signed-off-by: Lad, Prabhakar <prabhakar.csengg@gmail.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>