ath5k: add LED support for Acer Aspire One AO751h/AO531h

Add LED support for a Foxconn AR242X module, found on
the Acer Aspire One models AO751h/AO531h

Signed-off-by: Keng-Yu Lin <keng-yu.lin@canonical.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>