vyos-kernel: enable atheros wireless drivers in the helium 3.13 kernel

The current kernel (3.13) used in the helium branch of VyOS doesn't
include the ath5k and ath9k kernel modules, preventing the use of these
cards on such a system.  The current hydrogen release has support for
these cards, but the kernel modules don't correctly support newer
cards, or at least not this one (MikroTik R11e-2HPnD, based on the
AR9580 chipset).

Bug #263 http://bugzilla.vyos.net/show_bug.cgi?id=263