cpupower: Remove spurious return statement

print_duration() has a return; statement at the end of the function
that is not necessary as it's a void function.

Link: https://lore.kernel.org/r/20241218191144.3440854-2-superm1@kernel.org


Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>