hv: pass u32 to process_chn_event()

Change types in vmbus_on_event() to u32 since the input is u32 as well.
Pass u32 to process_chn_event() instead of casting arg to void* and back.
Update printk to reflect type change.

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>