intel_pstate: Fix type mismatch warning

The expression in line 398 of intel_pstate.c causes the following
warning to be emitted:

drivers/cpufreq/intel_pstate.c:398:3: warning: left shift count >= width of type

which happens because unsigned long is 32-bit on some architectures.
Fix that by using a helper u64 variable and simplify the code
slightly.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>