ieee80211: Print human-readable disassoc/deauth reason codes

Create a function to return a descriptive string for each reason code,
and print that in addition to the numeric value in the kernel log. These
codes are easily found on popular search engines, but one is generally
not able to access the internet when dealing with wireless connectivity
issues.

Signed-off-by: Calvin Owens <jcalvinowens@gmail.com>
[use 'unknown' rather than 'invalid' since more valid codes exist]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>