Use correct variable for init in GsmAlphabet

When initializing the shift table in GsmAlphabet the correct
variable is now used. An error was logged already if the value
is wrong, so this does not have any logic correctness or
performance impact, just code correctness.
Test: very small refactoring. Tests still pass.

Change-Id: I5329cfa0be63b6a3ebd06414c6247d94ca93f40b
Signed-off-by: Thom Hurks <thomhurks@gmail.com>