mwifiex: delete unused mwifiex_get_intf_num()

Commit 7afb94da ("mwifiex: update set_mac_address logic") fixed the
only user of this function, partly because the author seems to have
noticed that, as written, it's on the borderline between highly
misleading and buggy.

Anyway, no sense in keeping dead code around: let's drop it.

Fixes: 7afb94da

 ("mwifiex: update set_mac_address logic")
Signed-off-by: Brian Norris <briannorris@chromium.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>