usb: musb: only set test mode once

The MUSB test mode register can only be set once, otherwise the result
is undefined.

This prevents the debugfs testmode entry to set the register more than
once which causes test failure.

Signed-off-by: Bin Liu <b-liu@ti.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>
1 file changed