nl80211: clarify code in nl80211_del_station()

The long if chain of interface types is hard to read,
especially now with the additional condition after it.
Use a switch statement to clarify this code.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Link: https://lore.kernel.org/r/20200320113834.2c51b9e8e341.I3fa5dc3f7d3cb1dbbd77191d764586f7da993f3f@changeid


Signed-off-by: Johannes Berg <johannes.berg@intel.com>