wil6210: infrastructure for multiple virtual interfaces

Simple infrastructure changes for supporting multiple
virtual interfaces (multiple VIFs).
It is still not possible to add new VIFs so the only VIF
belongs to the main interface.
Main changes:
1. Add MAC ID(mid) argument to wmi_send and wmi_call to
allow invoking WMI commands on different VIFs.
2. Similarly, in WMI event handler look at the mid reported
by FW and extract VIF structure (currently only for main
interface). All WMI event handlers operate on wil6210_vif
structure so they know on which VIF they were called.
3. Trivial changes to use wil6210_vif structure and MID
throughout the code.
4. Various changes to logging to report MID.

More complete multiple VIFs support will be added gradually
in next patches.

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