icnss: Reduce the shutdown timeout from 50sec to 15sec

Sub-system shutdown timeout is 20 seconds. In wlan platform driver, 
modem block shutdown timeout should be less than sub-system timeout.
Hence, change block shutdown timeout to 15 seconds.

Change-Id: I936e0c462858727b86b35b924dd124a5d1434780
Signed-off-by: Mohammed Siddiq <msiddiq@codeaurora.org>