ath5k: Remove AHB bus support

AHB bus support was added in v2.6.38, through commit a0b907ee2a71
("ath5k: Add AHB bus support."). That code can only be build if the
Kconfig symbol ATHEROS_AR231X is set. But that symbol has never been
added to the tree. So AHB bus support has always been dead code.

Let's remove all code that depends on ATHEROS_AR231X. If that symbol
ever gets added to the tree the AHB bus support can be re-added too.

Signed-off-by: Paul Bolle <pebolle@tiscali.nl>
Signed-off-by: John W. Linville <linville@tuxdriver.com>