mhi: controller: always return success for suspend when modem is off

When external modem is on a powered off state, always return success
for suspend. Otherwise, MHI host driver prevent system suspend
from happening.

CRs-Fixed: 2319740
Change-Id: I8493742cd5855bb568286c74f50062f9d2e73efe
Signed-off-by: Sujeev Dias <sdias@codeaurora.org>