platform/x86: amd-pmc: Make amd_pmc_stb_debugfs_fops static

amd_pmc_stb_debugfs_fops is not used outside of amd-pmc.c, make it
static.

Cc: Sanket Goswami <Sanket.Goswami@amd.com>
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Link: https://lore.kernel.org/r/20220117112644.260168-1-hdegoede@redhat.com