platform/mellanox: Group create/destroy with attribute functions

Move the mlxreg_hotplug_device_create and _destroy functions up with the
related attribute functions. No functional changes.

Signed-off-by: Vadim Pasternak <vadimp@mellanox.com>
Acked-by: Andy Shevchenko <andy.shevchenko@gmail.com>
[dvhart: refactored commit into smaller functional changes]
Signed-off-by: Darren Hart (VMware) <dvhart@infradead.org>