ibmvscsi: Use of_root to access OF device tree root node

The root node of the OF device tree is exported as of_root. No need to
look up the root by path name. Instead just get a reference directly via
of_root.

Signed-off-by: Tyrel Datwyler <tyreld@linux.vnet.ibm.com>
Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>