Make sure to set the active_mtu attribute to avoid report the following
invalid value:

$ ibv_devinfo -d siw0 | grep active_mtu
			active_mtu:		invalid MTU (0)

Fixes: 303ae1cd ("rdma/siw: application interface")
Link: https://lore.kernel.org/r/20200205081354.30438-1-kamalheib1@gmail.comSigned-off-by: NKamal Heib <kamalheib1@gmail.com>
Reviewed-by: NGal Pressman <galpress@amazon.com>
Reviewed-by: NBernard Metzler <bmt@zurich.ibm.com>
Signed-off-by: NJason Gunthorpe <jgg@mellanox.com>