ISDN: remove uses of isdn_tty_revision

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...

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Cc: Karsten Keil <isdn@linux-pingi.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>