net: wireless: Add CONFIG_ATH6K_LEGACY for QCA AR6003

CONFIG_ATH6K_LEGACY is added to support QCA AR6003 (Non cfg80211).
AR6003 driver needs WIRELESS_EXT support. WIRELESS_EXT is going to
be deprecated so adding it to defconfig will not work. Driver needs
to mark it as dependency.

Change-Id: I09c1f07ea15b8de75c1ed35d4fec6c82265018a9
Acked-by: Prasanth Bhatta <c_bhatta@qca.qualcomm.com>
Signed-off-by: Rohit Vaswani <rvaswani@codeaurora.org>