Fixes issue where directory names are uppercase in MTP mount but lowercase in update file.

[daitj]

For example unit_filepath was Garmin/Text/french.ln4 in the update but then on my MTP mount it was found under GARMIN/TEXT/french.ln4

This should also create non existing directory like GARMIN/ExtData which was needed for some files from update.

[abadiet]

Thank you for the PR. However, the Device.get_updates()'s request is still not working properly. So, I prefer to fix this first, as the directory names error might be due to a malfunction in this request.