[POWERPC] spu_base: move spu_init_channels out of spu_mutex

There is no reason to execute spu_init_channels under spu_mutex
after the spu has been taken off the freelist it's ours.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Arnd Bergmann <arnd.bergmann@de.ibm.com>