staging: wilc1000: use vif index to communicate with wilc device

We now have vif index in all functions related with host interface thread.
wilc_get_vif_idx and wilc_get_vif_from_idx are added to get id and vif
respectively.
Relace get_id_from_handler with wilc_get_vif_idx and get_handler_from_id with
wilc_get_vif_from_idx. Remove unused function get_handler_from_id as well.
We get vif where wilc_get_vif_from_idx is called, so pass vif to msg.vif.

There are two get_id_from_handler left. They will be removed in later patch.
Signed-off-by: Glen Lee <glen.lee@atmel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>