Fix TPIDRPRW_IS_CURLWP builds.

A MULTIPROCESSOR kernel requires TPIDRPRW_IS_CURCPU.
Pull workqueue_destroy out of sme->sme_mtx

workqueue_destroy may sleep so we shouldn't run it with holding a mutex.

Requested by riastradh@.
Revert previous change to dwc_gmac_txintr() - while it looks strange at
first sight (and sorry I didn't spot it when reviewing), it is a small
optimization and actually correct.
Add a comment explaining it.