Make sure NWP is stopped before sl_Start

In SDK 1.2.0 TI decided to stop resetting NWP before sl_Start, which in
practice means that sl_start will hang on subsequent runs after the first.

See this post for details and suggested solution:

https://e2e.ti.com/support/wireless_connectivity/simplelink_wifi_cc31xx_cc32xx/f/968/p/499123/1806610#1806610

However, since they don't provide OS_debug variant of simplelink.a and
adding another project dependency will complicate our demo even more,
we just take the required bit of code.

PUBLISHED_FROM=c729c0fcbb240aa90a716edd43888de5881d6731