Summary:
mtp: Simplify code a bit

Only one branch can be taken per call, make this clear by using
if/else-if instead of a series of ifs.

mtp: Don't crash if getPath returns something invalid

This happens when you try to copy a file from a device which just got
locked again (I think...)

Reviewers: elvisangelaccio

Reviewed By: elvisangelaccio

Differential Revision: https://phabricator.kde.org/D6620