usb: dwc3: debugfs: hold the lock in during mode change

The read and write operation is atomic and we need no locking around
this operations. What we need however is a lock that is held which
ensures that the content of the DWC3_GCTL has not been changed. With
this, the conten may have been change changed after the first but before
our write back.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Felipe Balbi <balbi@ti.com>