staging: ion: Return correct type from map_kernel

This fixes the following issue:

[   41.488268] WARNING: at
.../kernel/drivers/staging/android/ion/ion.c:645
ion_buffer_kmap_get.part.9+0x44/0x84()
[   41.500673] heap->ops->map_kernel should return ERR_PTR on error

Change-Id: If6638619835ef7a2568a6b5c8ae0d80888470181
Signed-off-by: Olav Haugan <ohaugan@codeaurora.org>