Linux by Zabbix agent template cannot record interface speeds for interfaces that are down (represented by an interface speed of -1000000).

Steps to reproduce:

1. Apply `Linux by Zabbix agent` to a host.
2. Allow discovery to occur.
3. Note that some interfaces report (-1000000) as the interface speed. These interfaces are down.
4. Note that the Zabbix logs will say the item is unsupported due to string (or double, is inconsistent) value does not match Numeric (unsigned).
5. Interface speed is not recorded.

I have worked around this by adding a preprocessing step that replaces values of -1000000 with 0.