[CLEANUP] Improve code style for FormEngine::getTSCpid()

The PHPDoc is fixed. The method returns an array and not an integer.

Additionally the class variable that is used in this method for
storing the cached values is added as a property.

Releases: 6.2
Resolves: #56658
Change-Id: I6010cf97781001cd760c891da3df3bfb7a605b73
Reviewed-on: https://review.typo3.org/28167
Reviewed-by: Tymoteusz Motylewski
Tested-by: Tymoteusz Motylewski
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn