ieee80211: add definition for interworking support

IEEE802.11u interworking support is advertised via extended
capabilities IE bit 31. This is 7th bit of 4th byte of extended
capabilities.

Signed-off-by: Avinash Patil <patila@marvell.com>
Signed-off-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>