ath9k: Use proper TX channel width for setting channels

The TX channel width of the BSS can be obtained only after association.
In all cases, default to HT20 if HT is enabled, and set
chan width to HT40 only if the BSS supports it.

Signed-off-by: Sujith <Sujith.Manoharan@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>