Fix for softserial code.

"Bit end timer must by shifted by half-bit period to startbit edge, so that next edges are in middle of bit interval. This behavior was masked with old timer code, but noise immunity was probably very low."

-Backported from @ledvinap's code here. https://github.com/ledvinap/cleanflight/commit/3b20b74c0017fbc210a3091bf7ff4fbe42abfdf7#diff-0