cmd: usb_mass_storage: Staticize do_usb_mass_storage()

Make do_usb_mass_storage() static to fix the following sparse
warning:

cmd/usb_mass_storage.c:136:5: warning: symbol 'do_usb_mass_storage' was not declared. Should it be static?

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>

cmd/usb_mass_storage.c

@@ -133,7 +133,7 @@ cleanup:
 	return ret;
 }
 
-int do_usb_mass_storage(cmd_tbl_t *cmdtp, int flag,
+static int do_usb_mass_storage(cmd_tbl_t *cmdtp, int flag,
 			       int argc, char * const argv[])
 {
 	const char *usb_controller;