mac80211: add a couple of trace event classes to reduce duplicated code

The functions drv_add_interface() and drv_remove_interface() print out
the same values in the traces.  Combine the traces of these two
functions into one event class to remove some duplicate code.

Also add a new class for functions drv_set_frag_threshold() and
drv_set_rts_threshold().

Cc: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Luciano Coelho <coelho@ti.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>