net: tftp: fix tftp server initialization

Some globals where not properly initialized causing timeouts
as data packets where not immediately acknowledged.

Fixes: cc6b87ec ("net: tftp: Add client support for RFC 7440")
Signed-off-by: Arjan Minzinga Zijlstra <arjan.minzingazijlstra@fox-it.com>
Reviewed-by: Ramon Fried <rfried.dev@gmail.com>