cpupower: Fix bug where return value was not used

Save return value from amd_pci_get_num_boost_states
and remove redundant setting of *support

Signed-off-by: Sherry Hurwitz <sherry.hurwitz@amd.com>
Reviewed-by: Thomas Renninger <trenn@suse.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>