s390/hypfs_sprp: fix virtual vs physical address confusion

Add missing virt_to_phys() translation to __hypfs_sprp_diag304().
This doesn't fix a bug since virtual and physical addresses are
currently the same.

Reviewed-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>