kio-mtp: Better support for mtp devices

This patch removes the call of LIBMTP_Check_Specific_Device(), since
it is generally not adviced to use it at all, see [1]. Instead, it
should only be used in case LIBMTP_Detect_Raw_Devices() fails.
Looking into other mtp implementations (gvfs, smtpfs) revelas, that
LIBMTP_Check_Specific_Device() is not used at all. Therefore,
this patch brings the kio-mtp implementation more in line with
other implmentations.

It was confirmed [2] that with this patch some Windows Phones (e.g.
Lumia 630) get properly detected and work as expected.

  [1] https://sourceforge.net/p/libmtp/mailman/message/34948820/
  [2] https://www.reddit.com/r/kde/comments/4aggg0

REVIEW: 127386
BUG: 325335