WinCE: Fix reading of data from serial port
WinCE: Fix reading of data from serial port

Do not remove all read bytes from the input buffer,
but only the difference between the reserved ones
and the actual read ones.

(cherry-picked from 77c48a0e22c1048226832e6b845d72da9d5b317f)
Change-Id: Ib114e3972ec295f57d011ae9ffa6613313712e32
Reviewed-by: Denis Shienkov <6a9c747bd2ba446682324cf340d3b86f30cf4d55@gmail.com>

RoadWorksSoftware/QtSerialPort-OS2,RoadWorksSoftware/QtSerialPort-OS2