platform/mellanox: mlxreg-hotplug: Document fixes for hotplug private data

Add missing description of dev, regmap, dwork_irq, after_probe in struct
mlxreg_hotplug_priv_data.

Remove dwork field from the structure mlxreg_hotplug_priv_data itself and
for the descriptions, since it is not used.

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