usb: gadget: f_mass_storage: Fix sparse warning

Use NULL instead of 0 when returning pointer, to fix the following
sparse warnings.

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