cpufreq: amd-pstate: Make varaiable mode_state_machine static

smatch reports
drivers/cpufreq/amd-pstate.c:907:25: warning: symbol
  'mode_state_machine' was not declared. Should it be static?

This variable is only used in one file so it should be static.

Signed-off-by: Tom Rix <trix@redhat.com>
Reviewed-by: Wyes Karny <wyes.karny@amd.com>
Tested-by: Wyes Karny <wyes.karny@amd.com>
Reviewed-by: Dhruva Gole <d-gole@ti.com>
[ rjw: Subject edits ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>