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>