usb: gadget: move usb_gadget_controller_number() into a .c file and libcomposite

usb: gadget: move usb_gadget_controller_number() into a .c file and libcomposite

After I moved the function from the header file to the c file I see:

| $ size drivers/usb/gadget/gadget_chips.o
| text    data     bss     dec     hex filename
| 1048       0       0    1048     418 drivers/usb/gadget/gadget_chips.o

That is almost a KiB which is removed from each user.
As Felipe pointed out, the function / usage is very dumb actually. This is
used for the following reasons:
- epautoconf ep hint (could provide a per-gadget callback)
- miss-features. currently the missing altsetting on pxa's and something
  ZLP related on musbhdrc (looks like an optimisation which could be
  implemented in musb itself if it is correct)
- unique BCD accross all UDCs. Not sure how important this is.

Acked-by: Michal Nazarewicz <mina86@mina86.com>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Felipe Balbi <balbi@ti.com>