Make usb-stop() safe to call multiple times in a row.

Make usb-stop() safe to call multiple times in a row.

A recent commit (936897d4d1365452bbbdf8430db5e7769ef08d38)
enabled the usb_stop() command in common/cmd_bootm.c which was
not enabled for some time, because no board did actually set the
CFG_CMD_USB flag. So, now the usb_stop() is executed before
loading the linux kernel.

However, the usb_ohci driver hangs up (at least on AT91SAM) if the
driver is stopped twice (e.g. the peripheral clock is stopped on AT91).
If some other piece of code calls usb_stop() before the bootm command,
this command will hangup the system during boot.
(usb start and stop is typically used while booting from usb memory stick)

But, stopping the usb stack twice is useless anyway, and a flag already
existed that kept track on the usb_init()/usb_stop() calls.
So, we now check if the usb stack is really started before we stop it.

This problem is now fixed in both the upper as low-level layer.

Signed-off-by: Remy Bohmer <linux@bohmer.net>
Acked-by: Markus Klotzbuecher <mk@denx.de>