rndis_host: Use 1KB buffer in rndis_unbind

rndis_command requires the caller to pass in a buffer of at least 1KB.

Signed-off-by: Jussi Kivilinna <jussi.kivilinna@mbnet.fi>
Signed-off-by: Bjorge Dijkstra <bjd@jooz.net>
Acked-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>