ath(4): always enable 11n

Enabling 11n for ath(4) so far was handled by a kernel option, which
was only enabled for certain kernel configurations.
In order to allow loading ath(4) as a module with 11n support on
all platforms, remove the kernel option and unconditionally enable
11n in ath(4).

Reported by: pkubaj
Discussed with: adrian in D43549.
Reviewed by: adrian, imp
Differential Revision: https://reviews.freebsd.org/D43964