drivers: isdn: eicon: xdi_msg.h: Fix typo in #ifndef

Test for definedness of the macro which is actually defined (the
change is hard to see: it is s/SSS/SSA/).

Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Signed-off-by: David S. Miller <davem@davemloft.net>