amd-pstate-ut: Reset amd-pstate driver mode after running selftests

amd-pstate-ut: Reset amd-pstate driver mode after running selftests

In amd-pstate-ut, one of the basic test is to switch between all
possible mode combinations. After running this test the mode of the
amd-pstate driver is active mode. Store and reset the mode to its original
state.

Reviewed-by: Gautham R. Shenoy <gautham.shenoy@amd.com>
Signed-off-by: Swapnil Sapkal <swapnil.sapkal@amd.com>
Reviewed-by: Mario Limonciello <mario.limonciello@amd.com>
Link: https://lore.kernel.org/r/20250430064206.7402-1-swapnil.sapkal@amd.com
Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>