trivial: Don't assume that turbo_pct is present

According to kernel documentation this file is only exposed if the value will be the same for all CPUs.

"This attribute is present only if the value exposed by it is the same for all of the CPUs in the system." Link: https://www.kernel.org/doc/html/v6.9-rc1/admin-guide/pm/intel_pstate.html

I have no system to test if this is actually correct; I'm just going off documentation and trying to look for a possible root cause to https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1063757 which can still reproduce with 0.21.