usb: renesas_usbhs: Add a common function for the .get_id

All platform related codes (rcar[23].c and rza{2}.c) has its own
.get_id function as "USBHS_GADGET". So, we can use a common
function for it.

Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>