usb: gadget: push tty port allocation from gadget into f_acm

It possible to allocate the tty port number within the "instance"
structure of the function and there is no need to expose this
information within the gadget and therefore it is removed here.
This patch converts only f_acm and all its users. The other gadgets will
follow once the function is converted to the function interface.

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