net/enic: cap Rx packet processing to end of desc ring

net/enic: cap Rx packet processing to end of desc ring

In the default Rx handler stop processing packets at the end of
the completion ring so that wrapping doesn't have to be checked
in the inner while loop.

Also, check the color bit in the completion without using a conditional.

Signed-off-by: John Daley <johndale@cisco.com>
Reviewed-by: Hyong Youb Kim <hyonkim@cisco.com>