rndis_wlan: rename wireless stats worker to device poller

Stats worker no longer poll stats from device anymore. It's still
needed to poll device control channel for connect/disconnect events,
so rename stats worker as device poller.

Signed-off-by: Jussi Kivilinna <jussi.kivilinna@mbnet.fi>
Signed-off-by: John W. Linville <linville@tuxdriver.com>