usb: ehci: Remove unused ehci_{setup,shutdown}_phy() helpers

Remove unused ehci_{setup,shutdown}_phy() helpers now replaced by
generic_{setup,shutdown}_phy().

Signed-off-by: Patrice Chotard <patrice.chotard@foss.st.com>
Cc: Marek Vasut <marex@denx.de>
Cc: Simon Glass <sjg@chromium.org>