wil6210: add wil6210_vif structure for per-VIF data

wil6210: add wil6210_vif structure for per-VIF data

For supporting multiple virtual interfaces in the future,
introduce a wil6210_vif structure which will hold per-VIF
data. Change the module initialization so wil6210_vif will
be part of net_device structure, and wireless_dev will be
embedded inside the wil6210_vif structure. This will allow
us to find the appropriate wil6210_vif structure when we
only have access to wireless_dev or net_device.

Signed-off-by: Lior David <liord@codeaurora.org>
Signed-off-by: Maya Erez <merez@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>