ath9k: Reintroduce modparam to enable btcoex

It is not ideal to enable btcoex based on subsys id as it is
not unique, they are so random. It is also a pain keeping
all of them in a table to enable btcoex for a particular
hw. Going back to the old idea.

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