wil6210: rename p2p_wdev_mutex to vif_mutex

As more support is added for multiple VIFs, there is a need
to protect the wil6210_vif structure from access while it is
deleted (mainly from del_virtual_intf operation).
Instead of adding another mutex, use p2p_wdev_mutex and rename
it to vif_mutex to reflect the added role. Its existing roles
are similar so it extends nicely (for example it protects the
scan_request member which is also a member of wil6210_vif).

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