phy: freescale: imx8qm-hsio: fix NULL pointer dereference

In the Linux kernel, the following vulnerability has been resolved:

phy: freescale: imx8qm-hsio: fix NULL pointer dereference

During the probe the refclk_pad pointer is set to NULL if the

'fsl,refclk-pad-mode' property is not defined in the devicetree node. But

in imx_hsio_configure_clk_pad() this pointer is unconditionally used which

could result in a NULL pointer dereference. So check the pointer before to

use it.