loader: tftp: Don't error on tftp error 0

tftp-hpa sends NAK with tftp error set to 0 when trying to get
a directory and this is the first thing that loader tries to do
and this make it hangs.

Reviewed by: imp, tsoome
MFC after: 2 weeks
Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D33406

(cherry picked from commit bf07f2f8623d5e29e814b9dc49a0cdff920778c3)