RDMA/qedr: Remove unsupported modify_port callback

There is no need to return always zero for function which is not
supported.

Fixes: ac1b36e5 ("qedr: Add support for user context verbs")
Link: https://lore.kernel.org/r/20191028155931.1114-5-kamalheib1@gmail.com


Signed-off-by: Kamal Heib <kamalheib1@gmail.com>
Reviewed-by: Jason Gunthorpe <jgg@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>