usb: gadget: g_serial: split the three possible functions into three bind functions

This patch factors out the three possible functions into three possible
bind functions which are passed as an argument to usb_add_config(). This
will ease the step by step converting of the individual functions to the
new function registration method.

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