[ath] Add LDPC transmit support.

LDPC adds better transmit reliability if both ends support it.

You in theory can do both STBC and LDPC at the same time.
If I see issues I'll disable it.

• Only enable it if both ends of a connection negotiate it.
• Disable it if any rate is non-11n.
• Count both LDPC TX and STBC TX.

Tested:

• AR9380, STA mode