ath9k: Fix tx struck state with paprd

Paprd needs to be done only on active chains(not for all the chains
that hw can support). The paprd training frames which are sent
for inactive chains would be hanging on the hw queue without
getting transmitted and would make the connection so unstable.
This issue happens only with the hw which supports paprd cal(ar9003).

Signed-off-by: Vasanthakumar Thiagarajan <vasanth@atheros.com>
Cc: stable@kernel.org
Signed-off-by: John W. Linville <linville@tuxdriver.com>