usb: ehci: adjust to use platform_drivers to register/unregister driver

Since upstream commit aad06846 ("usb: ehci: Simplify platform driver
registration"), PLATFORM_DRIVER was replace by platform_drivers,
adjust the old implement to use platform_drivers to register or
unregister platform.

This is use to fix a compile warning that import by axxia SDK
commit 7048bd2a ("usb: xhci: Add CI13612A USB driver for
Axxia AXM55xx"):

In file included from drivers/usb/host/ehci-hcd.c:1325:0:
drivers/usb/host/ehci-ci13612.c:223:31: warning: ‘ci13612_ehci_driver’ defined but not used [-Wunused-variable]
 static struct platform_driver ci13612_ehci_driver = {
                               ^~~~~~~~~~~~~~~~~~~

Signed-off-by: Liwei Song <liwei.song@windriver.com>
Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>