usb: dwc3: use a helper function for operation mode setting

There are two where need to set operational mode:
- during initialization while we decide to run in host,device or DRD
  mode
- at runtime via the debugfs interface.

This patch provides a new function which sets the operational mode and
moves its initialiation to the mode switch instead in the gadget code
itself.

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