wil6210: protect against invalid length of tx management frame

This check is not valid:
if (len < sizeof(struct ieee80211_mgmt))
Because ieee80211_mgmt contains the ieee80211 header followed by
a union of various action frames, so the check will fail when trying
to send any management frame which is smaller than the largest action
frame in the union. This breaks FST and possibly other features.
Fix this by checking only against the header structure size.

Change-Id: I730300e180d9509f3555f16a0803af53cc8eca0a
Signed-off-by: Lior David <liord@codeaurora.org>