Staging: hv: check return value of driver_for_each_device()

The return value of driver_for_each_device() is now checked.  A
non-zero value simply generates a warning message, but it's better
than not checking at all.

Signed-off-by: Bill Pemberton <wfp5p@virginia.edu>
Cc: Hank Janssen <hjanssen@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>