cpufreq/amd-pstate: Fix the clamping of perf values

The clamping in freq_to_perf() is broken right now, as we first typecast
(read wraparound) the overflowing value into a u8 and then clamp it down.
So, use a u32 to store the >255 value in certain edge cases and then clamp
it down into a u8.

Also, use a "explicit typecast + clamp" instead of just a "clamp_t" as the
latter typecasts first and then clamps between the limits, which defeats
our purpose.

Fixes: 620136ce ("cpufreq/amd-pstate: Modularize perf<->freq conversion")
Signed-off-by: Dhananjay Ugwekar <dhananjay.ugwekar@amd.com>
Reviewed-by: Mario Limonciello <mario.limonciello@amd.com>
Link: https://lore.kernel.org/r/20250222033221.554976-1-dhananjay.ugwekar@amd.com


Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>