MAINTAINERS: update vmxnet3 driver maintainer

Shrikrishna Khare would no longer maintain the vmxnet3 driver. Taking
over the role of vmxnet3 maintainer.

Signed-off-by: Ronak Doshi <doshir@vmware.com>
Signed-off-by: Shrikrishna Khare <skhare@vmware.com>
Signed-off-by: David S. Miller <davem@davemloft.net>