[xbian-xbmc-cpufreq] Fix issues reported by @mk01.

* Moved CPUFREQ_GOVERNOR to an env, no need to test it for null as we
  already check for bad values just below it.
* Upstart executes scripts with `set -e`: the test for bad values didn't
  actually do nothing.  Oops.

Fixes #4.
Merge pull request #5 from anaconda/fix-xbmc-cpufreq

[xbian-xbmc-cpufreq] Fix issues reported in #4.