ath9k: Maintain monotonicity of PER while going across different phy

Monotonicity of packet error rate should be kept when moving
from one phy to another (legacy to ht, ht single stream to dual,
etc). Current code skips updating per for other phys.

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