powerpc/spufs: add a missing mutex_unlock

A mutex_unlock(&gang->aff_mutex) in spufs_create_context() is missing
in case spufs_context_open() fails.  As a result, spu_create syscall
and spu_get_idle() may block.

This patch adds the mutex_unlock.

Signed-off-by: Kou Ishizaki <kou.ishizaki@toshiba.co.jp>
Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
Acked-by: Andre Detsch <adetsch@br.ibm.com>