[posix] fix build errors on macOS (#5101)
[posix] fix build errors on macOS (#5101)

This commit fixes 3 implicit conversion changes signedness errors,
when trying to build the posix core in daemon mode on darwin.
lanyuwen/openthread,lanyuwen/openthread,chshu/openthread,abtink/openthread,chshu/openthread,srickardti/openthread,srickardti/openthread,srickardti/openthread,librasungirl/openthread,abtink/openthread,openthread/openthread,lanyuwen/openthread,chshu/openthread,chshu/openthread,openthread/openthread,abtink/openthread,lanyuwen/openthread,bukepo/openthread,jwhui/openthread,jwhui/openthread,lanyuwen/openthread,openthread/openthread,srickardti/openthread,abtink/openthread,bukepo/openthread,jwhui/openthread,jwhui/openthread,bukepo/openthread,librasungirl/openthread,bukepo/openthread,librasungirl/openthread,librasungirl/openthread,openthread/openthread,chshu/openthread,lanyuwen/openthread,chshu/openthread