phy: omap-usb2: use omap_usb_power_off to power off the PHY during probe

No functional change. Previously omap_control_phy_power() was used to power
off the PHY during probe. But once phy-omap-usb2 driver is adapted to
use syscon, omap_control_phy_power() cannot be used. Hence used
omap_usb_power_off to power off the PHY.

Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Acked-by: Roger Quadros <rogerq@ti.com>