enic: workaround A0 erratum

A0 revision ASIC has an erratum on the RQ desc cache on chip where the
cache can become corrupted causing pkt buf writes to wrong locations.  The s/w
workaround is to post a dummy RQ desc in the ring every 32 descs, causing a
flush of the cache.  A0 parts are not production, but there are enough of
these parts in the wild in test setups to warrant including workaround.  A1
revision ASIC parts fix erratum.

Signed-off-by: Scott Feldman <scofeldm@cisco.com>
Signed-off-by: David S. Miller <davem@davemloft.net>