usb gadget: don't save bind callback in struct usb_gadget_driver
To accomplish this the function to register a gadget driver takes the bind
function as a second argument. To make things clearer rename the function
to resemble platform_driver_probe.
This fixes many section mismatches like
WARNING: drivers/usb/gadget/g_printer.o(.data+0xc): Section mismatch in
reference from the variable printer_driver to the function
.init.text:printer_bind()
The variable printer_driver references
the function __init printer_bind()