Kconfig: USB: Migrate CONFIG_USB_EHCI to CONFIG_USB_EHCI_HCD

In order to be able to migrate the various SoC EHCI CONFIG options we
first need to finish the switch from CONFIG_USB_EHCI to
CONFIG_USB_EHCI_HCD.

Cc: Marek Vasut <marex@denx.de>
Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Marek Vasut <marex@denx.de>