atheros: v3.18: change calls logic

atheros: v3.18: change calls logic

Check SoC family (AR5312+ or AR2315+) before call instead of checking it
inside the called function. Also convert ar{5312,2315}_init_device()
function to void, since they both return zero and nobody care about
return value.

Signed-off-by: Sergey Ryazanov <ryazanov.s.a@gmail.com>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@44713 3c298f89-4303-0410-b956-a3cf2f4a3e73