mwifiex: reunite copy-and-pasted remove/reset code

When PCIe FLR code was added, it explicitly copy-and-pasted much of
mwifiex_remove_card() into mwifiex_shutdown_sw(). This is unnecessary,
as almost all of the code should be reused.

Let's reunite what we can for now.

The only functional changes for now:

 * call netif_device_detach() in the remove() code path -- this wasn't
   done before, but it really should be a no-op, when the device is
   getting totally unregistered soon anyway

 * call the ->down_dev() driver callback only after we've finished all
   SW teardown -- this should have no significant effect, since the only
   user (pcie.c) does very minimal work there, and it doesn't matter
   that we reorder this

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