WNM: Use CONFIG_WNM more consistently

WNM: Use CONFIG_WNM more consistently

Replace CONFIG_IEEE80211V with CONFIG_WNM to get more consistent build
options for WNM-Sleep Mode operations. Previously it was possible to
define CONFIG_IEEE80211V without CONFIG_WNM which would break the build.
In addition, IEEE 802.11v has been merged into IEEE Std 802.11-2012 and
WNM is a better term to use for this new functionality anyway.

Signed-hostap: Jouni Malinen <j@w1.fi>