MAINTAINERS: net: update Solarflare maintainers

Cc: Martin Habets <mhabets@solarflare.com>
Signed-off-by: Bert Kenward <bkenward@solarflare.com>
Acked-by: Martin Habets <mhabets@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>