cpufreq/amd-pstate: Use FIELD_PREP and FIELD_GET macros

The FIELD_PREP and FIELD_GET macros improve readability and help
to avoid shifting bugs.

Reviewed-by: Gautham R. Shenoy <gautham.shenoy@amd.com>
Link: https://lore.kernel.org/r/20241209185248.16301-7-mario.limonciello@amd.com


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