wil6210: add support for adding and removing virtual interfaces

Add generic support in cfg80211 operations add_virtual_intf
and del_virtual_intf for adding/removing VIFs of any
interface type, and fix change_virtual_intf to allow changing
the interface type of a VIF. Previously these operations
only worked for the P2P_DEVICE interface which is not a real
VIF(it is management-only and shares radio with the main
interface).
Currently the interface combination is validated, the VIF is
added/removed in the firmware and the appropriate net/wireless
device is also added/removed.
Added minimal support for proper interface up/down and module
unload but most operations still work only on the main interface.

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