Staging: hv: storvsc: Do not aquire an unnecessary reference on stor_device

On entry into storvsc_on_io_completion() we have already acquired a reference
on the stor_device; there is no need to acquire an additional reference 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>