Commit "ISDN: i4l, remove cvs crap" removed definition of
isdn_tty_revision, but there is still a user. So this causes linking
errors. This was hidden from my radar because the variable was not
declared in any header. Instead isdn_common.c declares it locally.
So remove this variable also from isdn_common.c, because there is
really no way to find out the version. Git commit or tag is...