if_ffec: free the dmamem allocation in detach

Calling bus_dmamap_destroy() for a mapping which was allocated with
bus_dmamem_alloc() will result in a panic. This change is not run-time
tested, but I identified the issue while implementing the analogous
method in if_dwc(4), using this implementation as the template.

MFC after: 1 week
Sponsored by: The FreeBSD Foundation