dsp: Update return value when ion_map_kernel fails

In ion_alloc, return value is not updated when ion_map_kernel fails.
This leads to access of invalid memory on failure.
Update return value in failure case to fix this.

Change-Id: Ib32a400a5fe9498f45ba0586978fe9ee3a6d827a
Signed-off-by: Vaishnavi Kommaraju <vkommara@codeaurora.org>