icnss: report fail to send modem shutdown message only when ret is negative

Report fail to send modem shutdown message only when return value is
negative as QMI framework will return positive value when qmi_txn_wait
returns success.

CRs-Fixed: 2283477
Change-Id: I30b18445f42cb329692f2a14a437b9555d917936
Signed-off-by: Yuanyuan Liu <yuanliu@codeaurora.org>