platform_data/mlxreg: Document fixes for hotplug device

Remove redunadant description of label in struct mlxreg_hotplug_device.

Change location of access_mode in struct mlxreg_hotplug_device.

Signed-off-by: Vadim Pasternak <vadimp@mellanox.com>
Signed-off-by: Darren Hart (VMware) <dvhart@infradead.org>