If we are in d0i3 when entering suspend, we leave d0i3 so that
mac80211 can call us to remove connections or whatever before going to
suspend.  We do this by calling pm_runtime_resume() early in the slave
transport flow and reactivating it later, when the wiphy suspend flow
runs.

The problem is that we queue a work in order to leave d0i3.  If this
work hasn't run yet when the wiphy suspend flow is called, we have a
race and entering d0i3 fails (because we're still holding the
IWL_MVM_REF_EXIT_WORK reference).

To solve this, simply flush the d0i3_exit_work at the beginning of the
iwl_mvm_suspend() function.
Signed-off-by: Luciano Coelho <luciano.coelho@intel.com>