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 is due to the fact that pstate is of type int.  Fix that by
using a helper unsigned long variable and simplify the code slightly.

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