RDMA/siw: Fix setting active_mtu attribute

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.com


Signed-off-by: Kamal Heib <kamalheib1@gmail.com>
Reviewed-by: Gal Pressman <galpress@amazon.com>
Reviewed-by: Bernard Metzler <bmt@zurich.ibm.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>