IB/umad: Avoid additional device reference during open()/close()

ib_umad_init_port_dev() holds the reference of a ib_umad_device instance.
ib_umad_device contains standard core device and cdev.  cdev holds the
reference of its parent core device.  file ops holds the reference to cdev
using core kernel.

Therefore, there is no need to hold additional reference while opening
umad related char devices.

While at it, add comments to bring clarity on releasing references to
ib_umd_device.

Signed-off-by: Parav Pandit <parav@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>