ath9k: Fix chainmask selection during scanning

The TX/RX chainmasks were set to 1x1 during scanning.
Configure them properly with the values retrieved from
the EEPROM.

Also, this requires scan_start/scan_end callbacks to be
locked with sc->mutex.

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