rndis_wlan: use kzalloc to allocate private data

rndis_wlan used kmalloc to allocate private data structure and leaving data
uninitialized, but later assumed to be set zero.

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