wil6210: add debugfs 'mids' file

Added a new debugfs file 'mids' to print the list of
virtual interfaces by MAC ID (MID). Allows mapping
between the internal MID used by FW and the actual
network interface used by the VIF. This is needed by
debugging tools.

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