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.