ath6kl: simplify btcoex parameter programming

Make the code more legible by parsing the config options on
the header file. While a it ensure to propagate errors and
bail out if we fail to set btcoex params.

Cc: Naveen Singh <nsingh@atheros.com>
Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>