collectd: fix smart disk detection

collectd: fix smart disk detection

On my system the attribute DEVTYPE was not set. The plugin could not
read any data and the function call blocked forever on this function and did
not returned. By removing it, all block devices under `/sys/class/block`
were checked.

Block devices that do not support SMART were not evaluated. The
collected displays the following message.

smart plugin: checking SMART status of /dev/loop4.
smart plugin: unable to open /dev/loop4.

If you do not like this message, you could only enable device in the uci that
does support SMART.

Signed-off-by: Florian Eckert <fe@dev.tdt.de>