mailbox: use error ret code of of_parse_phandle_with_args()

In case of error, of_parse_phandle_with_args() returns -EINVAL when the
passed index is negative, or -ENOENT when the index is for an empty
phandle. The mailbox core overwrote the error return code with a less
precise -ENODEV. Use the error returned code from
of_parse_phandle_with_args().

Signed-off-by: Tudor Ambarus <tudor.ambarus@linaro.org>
Signed-off-by: Jassi Brar <jassisinghbrar@gmail.com>