powerpc/rmu: Fix the error memory free parameters

There are error parameters should be corrected when
calling dma_free_coherent to free rmu rx-ring buffers
in fsl_open_inb_mbox() function.

Signed-off-by: Liu Gang <Gang.Liu@freescale.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Scott Wood <scottwood@freescale.com>