slim_msm: Handle external-modem shutdown differently than restart

External modem may shutdown in cases other than subsystem-restart.
In that case, the power-up notification may not happen.
Handle such a scenario and make sure runtime-pm vote is not held
for more time than required, and that slave-device up/down/reset
notifications happen as expected if slimbus was down.

CRs-Fixed: 645450
Change-Id: I5fef2393c835d06c5ce1dc4f9cb62d418620211b
Signed-off-by: Sagar Dharia <sdharia@codeaurora.org>