ath9k: move btcoex core driver info to its own struct

There is some bluetooth coexistance data which is driver
specific, stuff that into its own structure.

Cc: Vasanthakumar Thiagarajan <vasanth@atheros.com>
Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>