[POWERPC] 83xx: mpc832x_rdb: fix compiler warning

arch/powerpc/platforms/83xx/mpc832x_rdb.c: In function ‘mpc832x_rdb_setup_arch’:
arch/powerpc/platforms/83xx/mpc832x_rdb.c:104: warning: ‘np’ is used uninitialized in this function

Signed-off-by: Kim Phillips <kim.phillips@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>