rtwn: ensure TX work isn't scheduled during reset / abort

Don't schedule work during reset / abort. For USB NICs, work
must not be scheduled during a call to rtwn_usb_abort_xfers(),
as then it'll cause the call to usbd_transfer_drain() to hang.

This fixes a hang I've been seeing where the NIC hits a TX timeout
and then the reset/re-init path is called. If data is scheduled
to be transmitted in that window, the call to usbd_transfer_drain()
would hang and require a hard reboot to recover.

Differential Revision: https://reviews.freebsd.org/D47479