Fix hddtemp drive label for devices not directly under /dev

This is a small fix to the display of hddtemp drive label.

Before the code was assuming the drive name was directly in /dev (ie. /dev/sda1), but if hddtemp daemon was started with a drive path like /dev/disk/by-id/ata-XXXXXXXXXXXX, the label was truncated like this (last line in the screenshot):
![hddtemp label example before the fix](http://i.imgur.com/8gxR5ZI.png)

After the fix, we take the last path component, which looks a lot nicer:
![hddtemp label example after the fix](http://i.imgur.com/54E9OlV.png)