mwifiex: pcie: don't leak DMA buffers when removing

When PCIe FLR support was added, much of the remove/release code for
PCIe was migrated to ->down_dev(), but ->down_dev() is never called for
device removal. Let's refactor the cleanup to be done in both cases.

Also, drop the comments above mwifiex_cleanup_pcie(), because they were
clearly wrong, and it's better to have clear and obvious code than to
detail the code steps in comments anyway.

Fixes: 4c5dae59 ("mwifiex: add PCIe function level reset support")
Cc: <stable@vger.kernel.org>
Signed-off-by: Brian Norris <briannorris@chromium.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>