Minor updates to fix compile warnings for 11.3.1 toolchain

[mjs513]

@ftrias

Made some minor changes to get rid of warnings as a result of the toolchain update for Teenys: https://forum.pjrc.com/threads/71048-Teensyduino-1-58-Beta-1-(updated-toolchain-trial)?p=312338&viewfull=1#post312338.