soc: qcom: mem-offline: Check return value from mbox_send_msg properly

mbox_send_msg returns a value >= 0 on success, and a negative value
on failure.

Change-Id: I9fe56d5fddda0d6abb8dd5eb95743f12adac9eef
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>