usb: dwc3: move all string helper functions to debug.h

Those functions are only using within debugging
messages, grouping them into debug.h makes sense.

While at that, also add missing multiple inclusion
guard.

Signed-off-by: Felipe Balbi <balbi@ti.com>

diff --cc drivers/usb/dwc3/debug.h
Simple merge

diff --cc drivers/usb/dwc3/ep0.c
Simple merge

diff --cc drivers/usb/dwc3/gadget.c
Simple merge

diff --cc drivers/usb/dwc3/gadget.h
Simple merge