usb: dwc3: move generic dwc3 code from gadget into core

A few inits like the scale value or the removal of the DISSCRAMBLE is
done in the gadget code however it touches a general register.
Move this piece to the core.c file since it is likely to be requied by
both, parts of the core (device and host).

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