staging: wilc1000: change vif to pointer to refence real private data

vif of struct has it's own memory which is not necessary because we have
allocated vif from netdev_priv.
Change vif to pointer type and assign vif which is netdev private data.
Change it's operator on related codes as well.

Signed-off-by: Glen Lee <glen.lee@atmel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>