usb: storage: fix runtime pm issue in usb_stor_probe2

Since commit 71723f95 "PM / runtime: print error when activating a
child to unactive parent" I see the following error message:

scsi host2: usb-storage 1-3:1.0
scsi host2: runtime PM trying to activate child device host2 but parent
	    (1-3:1.0) is not active

Digging into it it seems to be related to the problem described in the
commit message for cd998ded "i2c: designware: Prevent runtime
suspend during adapter registration" as scsi_add_host also calls
device_add and after the call to device_add the parent device is
suspended.

Fix this by using the approach from the mentioned commit and getting
the runtime pm reference before calling scsi_add_host.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>