drivers: usb: fsl: Modify phy clk valid bit checking

Phy_clk_valid bit is checked only when the boolean
property phy-clk-valid in present in usb node device tree.
This property is added to the usb node via device tree fixup.

Signed-off-by: Nikhil Badola <nikhil.badola@freescale.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>