usb: gadget: gadgetfs: fix sparse warnings

this patch fixes following sparse warnings:

g_ffs.c:136:3: warning: symbol 'gfs_configurations' was not declared. Should it be static?
g_ffs.c:281:16: warning: Using plain integer as NULL pointer

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