Staging: hv: hv_mouse: rename hv_mouse_drv.c

As there's only one file for this driver, just name it the same
as the end module name, saving one build/link step and making it
simpler in the end.

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>