Staging: hv: mousevsc: Cleanup mousevsc_connect_to_vsp()

Cleanup mousevsc_connect_to_vsp(). There is no need to take reference on the
mousevsc device object when we are setting up the device. As part of this
cleanup get rid of get_input_device() as this function is only used here.

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