enic: organize device initialization/deinit into separate functions

To unclutter probe() a little bit, put all device initialization code
in one spot and device deinit code in another spot.  Also remove unused
rq->buf_index variable/func.

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