mwifiex: delete unused mwifiex_get_intf_num()

commit 1c9f329b084b7b8ea6d60d91a202e884cdcf6aae upstream.

Commit 7afb94da3cd8 ("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: 7afb94da3cd8 ("mwifiex: update set_mac_address logic")
Signed-off-by: Brian Norris <briannorris@chromium.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>