usb: ehci-mx6: remove unnecessary regulator enable from probe

Remove the regulator_set_enable() call from device probe which
resolves a regulator imbalance. This is unnecessary as
regulator_set_enable() will be called when ehci_register calls the
init_after_reset hook.

Suggested-by: Marek Vasut <marex@denx.de>
Signed-off-by: Tim Harvey <tharvey@gateworks.com>