Improve QSPI sysfs algorithm

Earlier, the algorithm used check for magic keywords (like "usb",
"platform" and so on) in the target path for decision-making what
of device can be skipped or accepted. But this is a wrong approach,
because on different platforms can be different key-words and we
can not know it. So, it is more reasonable to use a different
approach where we make decision by a driver name (by analogy with
current udev implementation). In this case we skip those devices
which has an empty driver name or when a name is equal to
"serial8250".

* When a driver name is empty, then we can be sure that it is
other than serial port device (e.g. as pseudo-terminal, console
and so on). Thus, it is similar to udev devices without a parent,
which we ignore.

* When a driver name is "serial8250", it is equal to "platform"
devices, which we too ignore for current implementation. Though in
a future this can be solved (i.e. we can try to detect a valid
devices based on "serial8250" driver).

Tested on Linux with on-board and USB serial ports using Qt5.

Change-Id: I884ad8f3fddff9487ad4a67c4904b57f62164e00
Reviewed-by: Rainer Keller <rainer.keller@theqtcompany.com>
Reviewed-by: Sergey Belyashov <Sergey.Belyashov@gmail.com>