platform/x86: mlx-platform: Add hotplug device unregister to error path

Add hotplug platform driver un-registration in case regmap cache
synchronization failed. In such case hotplug platform driver
registration should be rolled back.

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