usb: ehci: Simplify platform driver registration

Use platform_register_drivers() and platform_unregister_drivers() to
register and unregister ehci platform drivers. This simplifies the code
and prevents the following build errors seen with sparc:allmodconfig.

drivers/usb/host/ehci-hcd.c:1301: error:
	"PLATFORM_DRIVER" redefined
drivers/usb/host/ehci-sh.c:173:31: error:
	'ehci_hcd_sh_driver' defined but not used

Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/20210907123002.3951446-1-linux@roeck-us.net
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>