mwifiex: don't drop lock between list-retrieval / list-deletion

mwifiex_exec_next_cmd() seems to have a classic TOCTOU race, where we
drop the list lock in between retrieving the next command and deleting
it from the list. This potentially leaves room for someone else to also
retrieve / steal this node from the list (e.g.,
mwifiex_cancel_all_pending_cmd()).

Let's keep holding the lock while we do our 'ps_state' sanity checks.
There should be no harm in continuing to hold this lock for a bit more.

Noticed only by code inspection.

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