powerpc/papr_scm: use dev_get_drvdata

Eliminate direct accesses to the driver_data field.

Signed-off-by: Haowen Bai <baihaowen@meizu.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/1653988790-19999-1-git-send-email-baihaowen@meizu.com