rndis_host: pass buffer length to rndis_command

Pass buffer length to rndis_command so that rndis_command can read full
response buffer from device instead of max CONTROL_BUFFER_SIZE bytes.

Signed-off-by: Jussi Kivilinna <jussi.kivilinna@mbnet.fi>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>