cmd: CMD_USB depends on USB

The usb command cannot be compiled with CONFIG_USB=n:

ld.bfd: cmd/usb.c:660: undefined reference to `usb_stop'
ld.bfd: cmd/usb.c:663: undefined reference to `usb_started'

Reviewed-by: Marek Vasut <marex@denx.de>
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>