USB: musb: gadget: set otg tranceiver to idle when registering gadget

When registering gadget driver, the state of the transceiver
must be set from undefined (no gadget) to b_idle.

Module unload sets the transceiver state to undefined state.
After the first load/unload pair, the reset irq will be lost.

Signed-off-by: Arnaud Mandy <ext-arnaud.2.mandy@nokia.com>
Signed-off-by: Felipe Balbi <felipe.balbi@nokia.com>
Cc: David Brownell <dbrownell@users.sourceforge.net>
Acked-by: Anand Gadiyar <gadiyar@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>