Disable automatic header precompilation for QtSerialPort

There are two source files for this module only. Adding a header
precompilation is more expensive than the benefit.

Change-Id: I27f0756693ec655f56a781f94d14840ce48f71b4
Reviewed-by: Sergey Belyashov <Sergey.Belyashov@gmail.com>
Refactor unix setBaudRate() error handling

Return QSerialPort::SerialPortError instead of bool

Change-Id: I79d731c1a0178ef0238495d7e9bb3e1b866e82c0
Reviewed-by: Sergey Belyashov <Sergey.Belyashov@gmail.com>
Reviewed-by: Denis Shienkov <denis.shienkov@gmail.com>