MAINTAINERS: update tg3 maintainer list

Signed-off-by: Andy Gospodarek <gospo@broadcom.com>
Signed-off-by: Pavan Chebbi <pavan.chebbi@broadcom.com>
Signed-off-by: Siva Reddy Kallam <siva.kallam@broadcom.com>
Signed-off-by: Prashant Sreedharan <prashant.sreedharan@broadcom.com>
Reviewed-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>