mesh: Fix string signedness issue
mesh: Fix string signedness issue

Some compilers (like icx) may complain about unsigned vs signed strings.

X-Original-Commit: f544190b63cd2612d2e9aeb73af2c98793aa8834

apache/mynewt-nimble,apache/mynewt-nimble,apache/mynewt-nimble,apache/mynewt-nimble,apache/mynewt-nimble