usb: gadget: storage: fix sparse warning

fsg_common_set_inquiry_string() expects pointers
as second and third argument. Let's fix that by
passing NULL instead of plain 0 just so we silence
sparse's:

drivers/usb/gadget/f_mass_storage.c:3114:60: warning: \
	Using plain integer as NULL pointer
drivers/usb/gadget/f_mass_storage.c:3114:63: warning: \
	Using plain integer as NULL pointer

Signed-off-by: Felipe Balbi <balbi@ti.com>