ath6kl: remove AR600x_CUSTOM_XXX define

There already is a CONFIG_AR600x_CUSTOM_XXX if this is
desired to be used.

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