Staging: hv: Move the definition of the function get_stor_device()

In preparation for further cleaning up storvsc.c move the definition
of the inline function get_stor_device() from storvsc.c to
storvsc_api.h.

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