platform/x86: mlx-platform: mlxcpld-hotplug driver style fixes

The patch contains several styling fixes:
  - Make names of hotplug devices shorter;
  - Change register offset assignment to defines;
  - Add defines for the all event masks;
  - Use PLATFORM_DEVID_NONE instead of -1;

Signed-off-by: Vadim Pasternak <vadimp@mellanox.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>