Staging: hv: hv_mouse: unwind the initialization process a bit

This unwinds the init call sequence a bit, as we don't need a callback
pointer for a function that is already in this file.

Cc: Hank Janssen <hjanssen@microsoft.com>
Cc: K. Y. Srinivasan <kys@microsoft.com>
Cc: Haiyang Zhang <haiyangz@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>