luci-mod-status: luci-bwc: skip "." and ".." when finding netdevs

Skip the "." and ".." directory entries of "/sys/class/net/" instead of
passing them on to libiwinfo's probe function. This avoids a situation
where the kernel requests a modprobe of "netdev-." and "netdev-.." over
and over again, resulting in high load on lower end systems.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>