net/smc: rename connection index to RMBE index

The connection index is actually a RMBE index. So, this patch changes
the name accordingly.

Signed-off-by: Hans Wippel <hwippel@linux.ibm.com>
Signed-off-by: Ursula Braun <ubraun@linux.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>