ath9k: convert isWwrSKU macro into C code

Write isWwrSKU as an inline function and nix the camel-case to make
the routine slightly clearer.  Change its argument to take the regd
value directly so it can eventually be used by ath5k as well.

Signed-off-by: Bob Copeland <me@bobcopeland.com>
Acked-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>