ath6kl: use CONFIG_AR600x_WB31_XXX

When CONFIG_AR600x_WB31_XXX gets enabled we define
another flag called AR600x_WB31_XXX, just use
CONFIG_AR600x_WB31_XXX since that is already defined
by the generated autoconf header file from the kernel.

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