tcpdump: add STBC Rx support

Information will be represented as:
- Rx-STBC0 - mean, Rx-STBC is supported by driver but packet was not encoded in STBC
- Rx-STBC1 - 1 Nss stream was encoded in STBC
- Rx-STBC2 - 2 Nss streams
- Rx-STBC3 - 3

Signed-off-by: Oleksij Rempel <linux@rempel-privat.de>
add test for rx-stbc flag

Signed-off-by: Oleksij Rempel <linux@rempel-privat.de>
Merge pull request #318 from olerem/stbc

STBC Rx support and a new test case for it
justify the previous 802.11 test case name