cpufreq/amd-pstate: Only update the cached value in msr_set_epp() on success

If writing the MSR MSR_AMD_CPPC_REQ fails then the cached value in the
amd_cpudata structure should not be updated.

Reviewed-by: Gautham R. Shenoy <gautham.shenoy@amd.com>
Link: https://lore.kernel.org/r/20241209185248.16301-8-mario.limonciello@amd.com


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