MAINTAINERS: update cxgb4 and cxgb3 maintainer

Vishal Kulkarni will be the new maintainer for Chelsio cxgb3/cxgb4
drivers.

Signed-off-by: Arjun Vynipadath <arjun@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>