Replace all * portTICK_RATE_MS and /portTICK_RATE_MS by MS2TICKS and TICKS2MS macros

This has been bothering me for a while, and https://github.com/TauLabs/TauLabs/pull/479#discussion_r5308521 pushed me over the top.

There were lots of errors of this type, but because we were always dividing or multiplying by 1 it didn't produce any bugs. By moving to the macro, we can prevent future confusion over this portTICK_RATE_MS define.