Remove the unnecessary Q_DECLARE_PUBLIC from the data class

Change-Id: I8293dfc6d492bac59fa830d499902cabdc8d95dd
Reviewed-by: Sergey Belyashov <Sergey.Belyashov@gmail.com>
Reviewed-by: Denis Shienkov <denis.shienkov@gmail.com>
Remove the const modifier from the pinoutSignals internal methods

Logically, these methods set the error on their member, the public QSerialPort
class, hence these are not const methods similarly to the public variant.

Change-Id: I1580cf371556c820ce8e7df43121bc2800b08b40
Reviewed-by: Sergey Belyashov <Sergey.Belyashov@gmail.com>
Reviewed-by: Denis Shienkov <denis.shienkov@gmail.com>