mac80211: add dependency of PACKAGE_MAC80211_DEBUGFS

I wanted to use some statistics from /sys/kernel/debug/ieee80211,
so I enabled PACKAGE_MAC80211_DEBUGFS.  But I could not use it,
because CONFIG_KERNEL_DEBUG_FS was not set and it's a little bit
hidden. After manually setting CONFIG_KERNEL_DEBUG_FS to y,
everything was fine. This patch automatically selects
CONFIG_KERNEL_DEBUG_FS if PACKAGE_MAC80211_DEBUGFS is set.

Additionally, the help text says "Say N unless you know you need this.",
but it's default y. This help text is copied verbatim from the
kernel's Kconfig and not true for us. So remove this part of the
help text

Signed-off-by: Stefan Hellermann <stefan@the2masters.de>

git-svn-id: svn://svn.openwrt.org/openwrt@35014 3c298f89-4303-0410-b956-a3cf2f4a3e73