ath9k_htc: Add a modparam to enable BTCOEX rather than default

Enable BTCOEX for WB193(which seems to be the only supported
ath9k_htc BTCOEX chipset)only when it is enabled via modparam,
rather than enabling it by default.

Cc: Vivek Natarajan <nataraja@qualcomm.com>
Signed-off-by: Mohammed Shafi Shajakhan <mohammed@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>