It looks like at least tshark 2.6.3 uses a different error message for
unknown display filter fields:
tshark: Neither "wlan_mgt.fixed.category_code" nor "4" are field or protocol names.
and a different status exit code (2 instead of 1).
Add a new handler for this combination to allow automatic wlan_mgt to
wlan conversion to happen.
Signed-off-by: Jouni Malinen <j@w1.fi>