slim_msm: Gracefully handle QMI timeout failures

QMI timeout can happen if other processor doesn't respond within the
time expected by slimbus driver on apps. It has been noticed that QMI
call to power-on slimbus rarely takes more than 500msec. Timeout
is increased to allow that possibility. Additionally, QMI channel is
drained out of left-over messages and runtime-PM state check is
modified to allow graceful handling of timeout if at all any timeout
still happens.

Change-Id: I392a4fa761735135a261fb543815823cee5eba41
Signed-off-by: Sagar Dharia <sdharia@codeaurora.org>