staging: ion: Return correct value 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: I900d2adb4c009644f08bbcf6792acac034dc4b51
Signed-off-by: Olav Haugan <ohaugan@codeaurora.org>