media: mgb4: protect driver against spectre

In the Linux kernel, the following vulnerability has been resolved: media: mgb4: protect driver against spectre Frequency range is set from sysfs via frequency_range_store(), being vulnerable to spectre, as reported by smatch: drivers/media/pci/mgb4/mgb4_cmt.c:231 mgb4_cmt_set_vin_freq_range() warn: potential spectre issue 'cmt_vals_in' [r] drivers/media/pci/mgb4/mgb4_cmt.c:238 mgb4_cmt_set_vin_freq_range() warn: possible spectre second half. 'reg_set' Fix it.