cpufreq/amd-pstate-ut: Drop SUCCESS and FAIL enums

Enums are effectively used as a boolean and don't show
the return value of the failing call.

Instead of using enums switch to returning the actual return
code from the unit test.

Reviewed-by: Gautham R. Shenoy <gautham.shenoy@amd.com>
Reviewed-by: Dhananjay Ugwekar <dhananjay.ugwekar@amd.com>
Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>