Update USBAPI.h

Fixes bug where Serial.read() would always return 0 as the first byte.
Merge pull request #2124 from embmicro/patch-1

Update USBAPI.h
Fixed regression in USBAPI and CDC (xaljox)

See:
https://github.com/arduino/Arduino/commit/9ac7e30252fce5b9dafde3288519b020b73c37bd#commitcomment-6718676
Updates to revision log