usb: ehci-mx6: Use regulator_set_enable_if_allowed

usb: ehci-mx6: Use regulator_set_enable_if_allowed

With the commit 4fcba5d556b4 ("regulator: implement basic reference
counter") the return value of regulator_set_enable may be EALREADY or
EBUSY for fixed/gpio regulators.

Change to use the more relaxed regulator_set_enable_if_allowed to
continue if regulator already was enabled or disabled.

Based on 335799b7252a ("usb: dwc2: Use regulator_set_enable_if_allowed")

Signed-off-by: Marek Vasut <marex@denx.de>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Reviewed-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>