usb: gadget: f_mass_storage: use NULL instead of 0

The local variables such as 'filename', 'vendor_name', and
'product_name' are pointers; thus, use NULL instead of 0 to fix
the following sparse warnings

drivers/usb/gadget/f_mass_storage.c:3046:27: warning: Using plain integer as NULL pointer
drivers/usb/gadget/f_mass_storage.c:3050:28: warning: Using plain integer as NULL pointer
drivers/usb/gadget/f_mass_storage.c:3051:29: warning: Using plain integer as NULL pointer

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