Refactor QSerialPortPrivate::setBaudRate()

Split platform specific functions into their own setCustomBaudRate() methods to
improve readability.

As a side-effect, remove isCustomBaudRateSupported flag.

Change-Id: Ia26d5472219756fbf186c78aa049bba72966d7b5
Reviewed-by: Denis Shienkov <denis.shienkov@gmail.com>
Reviewed-by: Sergey Belyashov <Sergey.Belyashov@gmail.com>
QNX: Enable custom baud rate

Change-Id: Iec4124886514c5bdeae1cf00e3b2e9432a2c75ee
Reviewed-by: Sergey Belyashov <Sergey.Belyashov@gmail.com>