platform/mellanox: mlxreg-hotplug: Add check for negative adapter number

Verify before creation of hotplug device if the associated adapter number
is negative. It could be in case hotplug event is not associated with
hotplug device.

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