net: wireless: Rename CONFIG_ATH6K_LEGACY to CONFIG_ATH6K_LEGACY_EXT

CONFIG_ATH6K_LEGACY already exists in kernel tree so renaming to
CONFIG_ATH6K_LEGACY_EXT to support QCA AR6003 (Non cfg80211) driver
which will be compiled outside of kernel tree. AR6003 driver needs
WIRELESS_EXT support. WIRELESS_EXT is going to be deprecated so
adding it to defconfig will not work.

Change-Id: I171a3e4dbdf9bd2cf7e7eaa3741ec2fa9730a114
Signed-off-by: Sridhar Parasuram <sridhar@codeaurora.org>