rndis_host: Add early_init function pointer to 'struct rndis_data'.

Function pointer is for 'subminidrivers' that need to do work on device
right after minidriver has initialized hardware.

For example, rndis_wlan setting device specific configuration parameters
with OID_GEN_RNDIS_CONFIG_PARAMETER right after rndis_host has
initialized hardware with RNDIS_INIT.

Signed-off-by: Jussi Kivilinna <jussi.kivilinna@mbnet.fi>
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>