staging: android: ion: return -ENOMEM in ion_cma_heap allocation failure

Initial Commit 349c9e13 ("gpu: ion: add CMA heap") returns -1 in allocation
failure. The returned value is passed up to userspace through ioctl. So user can
misunderstand error reason as -EPERM(1) rather than -ENOMEM(12).

This patch simply changed this to return -ENOMEM.

Signed-off-by: Jaewon Kim <jaewon31.kim@samsung.com>
Acked-by: Laura Abbott <labbott@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>