mwifiex: fixup init_channel_scan_gap error case

In reading through _mwifiex_fw_dpc(), I noticed that after we've
registered our wiphy, we still have error paths that don't free it back
up. Let's do that.

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