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>
diff --git a/cmd/Kconfig b/cmd/Kconfig
index eff238c..863b7f9 100644
--- a/cmd/Kconfig
+++ b/cmd/Kconfig
@@ -1309,6 +1309,7 @@
 
 config CMD_USB
 	bool "usb"
+	depends on USB
 	select HAVE_BLOCK_DEVICE
 	help
 	  USB support.