{nl,cfg,mac}80211: Allow user to see/configure HT protection mode

This patch introduces a new mesh configuration parameter "ht_opmode" and will
allow user to check the current HT protection mode selected. Users could
configure the protection mode by the command "iw mesh_iface set mesh_param
mesh_ht_protection_mode=2". The default protection mode of mesh is set to
non-HT mixed mode.

Signed-off-by: Ashok Nagarajan <ashok@cozybit.com>
Reviewed-by: Thomas Pedersen <thomas@cozybit.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>