ieee80211: add the value for Category '6' in "rtw_ieee80211_category"

Preparation work for removing the "enum rtw_ieee80211_category" in
"drivers/staging/rtl8188eu/include/ieee80211.h" and
"drivers/staging/rtl8723bs/include/ieee80211.h".

This enum is similar to "enum ieee80211_category" from
"include/linux/ieee80211.h". However it defines the value '6' as
RTW_WLAN_CATEGORY_FT.

So add a corresponding value in "ieee80211_category"

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Link: https://lore.kernel.org/r/66be0187869bd7dae1c0b0785a32db695ee9872e.1624108556.git.christophe.jaillet@wanadoo.fr
Signed-off-by: Johannes Berg <johannes.berg@intel.com>