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>

drivers/usb/gadget/f_mass_storage.c

@@ -3043,12 +3043,12 @@ fsg_config_from_params(struct fsg_config *cfg,
 		lun->filename =
 			params->file_count > i && params->file[i][0]
 			? params->file[i]
-			: 0;
+			: NULL;
 	}
 
 	/* Let MSF use defaults */
-	cfg->vendor_name = 0;
-	cfg->product_name = 0;
+	cfg->vendor_name = NULL;
+	cfg->product_name = NULL;
 
 	cfg->ops = NULL;
 	cfg->private_data = NULL;