powerpc: handle error case in cpm_muram_alloc()

rh_alloc() returns (unsigned long)-ERRxx on error, which may
result in overwriting memory outside the MURAM AREA.

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Scott Wood <scottwood@freescale.com>