nl80211: correct checks for NL80211_MESHCONF_HT_OPMODE value

Previously, NL80211_MESHCONF_HT_OPMODE validation rejected correct
flag combinations, e.g. IEEE80211_HT_OP_MODE_PROTECTION_NONHT_MIXED |
IEEE80211_HT_OP_MODE_NON_HT_STA_PRSNT.

Doing just a range-check allows setting flags that don't exist (0x8)
and invalid flag combinations.

Implements some checks based on IEEE 802.11 2012 8.4.2.59 "HT
Operation element".

Signed-off-by: Masashi Honma <masashi.honma@gmail.com>
[reword commit message, simplify a bit]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>