Update softSerial.cpp - Use of auto -> Didn´t compile

[SirSundays]

Changed "auto start = millis()" to "unsigned long start = millis()"

It wouldn´t compile. Arduino complained that start was never defined.

So I looked at why and it had something to do with auto. I think auto is not needed here. The Arduino documentation says that millis() will return an unsigned long.

Arduino Reference millis: https://www.arduino.cc/reference/en/language/functions/time/millis/