ibmvnic: update MAINTAINERS

Update supporters for IBM Power SRIOV Virtual NIC Device Driver.
Thomas Falcon is moving on to other works. Dany Madden, Lijun Pan
and Sukadev Bhattiprolu are the current supporters.

Signed-off-by: Dany Madden <drt@linux.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>