RDMA/srpt: Fix disabling device management

RDMA/srpt: Fix disabling device management

Avoid disabling device management for devices that don't support
Management datagrams (MADs) by checking if the "mad_agent" pointer is
initialized before calling ib_modify_port, also fix the error flow in
srpt_refresh_port() to disable device management if
ib_register_mad_agent() fail.

Fixes: 180da608afe4 ("RDMA/srpt: Fix handling of SR-IOV and iWARP ports")
Link: https://lore.kernel.org/r/20200514114720.141139-1-kamalheib1@gmail.com
Signed-off-by: Kamal Heib <kamalheib1@gmail.com>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>