usb: musb: fix oops on musb_gadget_pullup

an 'unhandled fault' is causes when a gadget driver calls
usb_gadget_connect() while the USB cable isn't plugged into
the OTG port.

the fault is caused by an access to MUSB's memory space
while its clock is turned off due to pm_runtime kicking
in.

in order to fix the fault, we enclose musb_gadget_pullup()
with pm_runtime_get_sync() ... pm_runtime_put() calls to
be sure we will always reach that path with clock turned on.

[ balbi@ti.com : simplified commit log; removed few things
	which didn't belong there ]

Cc: stable@kernel.org
Reported-by: NZach Pfeffer <zach.pfeffer@linaro.org>
Signed-off-by: NJohn Stultz <john.stultz@linaro.org>
Signed-off-by: NFelipe Balbi <balbi@ti.com>

drivers/usb/musb/musb_gadget.c

@@ -1698,6 +1698,8 @@ static int musb_gadget_pullup(struct usb_gadget *gadget, int is_on)
 
 	is_on = !!is_on;
 
+	pm_runtime_get_sync(musb->controller);
+
 	/* NOTE: this assumes we are sensing vbus; we'd rather
 	 * not pullup unless the B-session is active.
 	 */
@@ -1707,6 +1709,9 @@ static int musb_gadget_pullup(struct usb_gadget *gadget, int is_on)
 		musb_pullup(musb, is_on);
 	}
 	spin_unlock_irqrestore(&musb->lock, flags);
+
+	pm_runtime_put(musb->controller);
+
 	return 0;
 }