xkb: fix XkbSetMap check for the keytypes count

The previous if/else condition resulted in us always setting the key
type count to the current number of key types. Split this up correctly.

Regression introduced in de940e06

Fixes #1249

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
(cherry picked from commit be16bd85)