ath9k: Cleanup WLAN/BT RX diversity

For single-chain WLAN+BT cards, the BT antenna can be used for
WLAN RX when the BT interface is disabled. Rename the modparam
"antenna_diversity" to "bt_ant_diversity" to clarify this.

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