cfg80211: make WMM TSPEC support flag an nl80211 feature flag

During the review of the corresponding wpa_supplicant patches we
noticed that the only way for it to detect that this functionality
is supported currently is to check for the command support. This
can be misleading though, as the command was also designed to, in
the future, support pure 802.11 TSPECs.

Expose the WMM-TSPEC feature flag to nl80211 so later we can also
expose an 802.11-TSPEC feature flag (if needed) to differentiate
the two cases.

Note: this change isn't needed in 3.18 as there's no driver there
yet that supports the functionality at all.

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