mwifiex: fix use-after-free for FW reinit errors

If we fail to reinit the FW when resetting the device (in the
synchronous version of mwifiex_init_hw_fw() -> mwifiex_fw_dpc()),
mwifiex_fw_dpc() will tear down the interface and free up the adapter.
But we don't actually check for all failure cases of mwifiex_fw_dpc(),
so some of them fall through and dereference adapter->fw_done with a
freed adapter, causing a use-after-free bug.

In any case, mwifiex_fw_dpc() will always signal FW completion -- in the
error OR success case -- so at best, this was repeat work. Let's not do
it.

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