mwifiex: don't leak 'chan_stats' on reset

'chan_stats' is (re)allocated in _mwifiex_fw_dpc() ->
mwifiex_init_channel_scan_gap(), which is called whenever the device is
initialized -- at probe or at reset.

But we only free it in we completely unregister the adapter, meaning we
leak a copy of it during every reset.

Let's free it in the shutdown / removal paths instead (and in the
error-handling path), to avoid the leak.

Ideally, we can eventually unify much of mwifiex_shutdown_sw() and
mwifiex_remove_card() (way too much copy-and-paste) to reduce the burden
on bugfixes like this. But that's work for tomorrow.

Signed-off-by: Brian Norris <briannorris@chromium.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>