usb: core: comply to PHY framework

Current implementation of the USB core does not take into account the
new PHY framework. Correct the situation by adding a call to
phy_set_mode() before phy_power_on().

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>