if_iwx: use net80211 suspend and resume methods

Use net80211 suspend and resume methods when entering and leaving
suspend. Move the resume action outside of the IWX_LOCK.

Re-assert disabling the PCI timeout, this is copied from OpenBSD (and a
common pattern), but it doesn't seem to hurt.

PR: 286059
Reviewed by: adrian
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D50462