bus_dmamap_sync the rx ring once per em_rxeof call, rather than for every rx descriptor.
bus_dmamap_sync the rx ring once per em_rxeof call, rather than for every
rx descriptor.

slightly tweak by matthew
tested on alpha and amd64

orumin/openbsd-efivars,orumin/openbsd-efivars,orumin/openbsd-efivars,orumin/openbsd-efivars