net: tftp: fix tftp server initialization

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

Fixes: cc6b87ecaa96 ("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>
diff --git a/net/tftp.c b/net/tftp.c
index e1e3597..bfc4c9b 100644
--- a/net/tftp.c
+++ b/net/tftp.c
@@ -906,6 +906,8 @@
 	tftp_block_size = TFTP_BLOCK_SIZE;
 	tftp_cur_block = 0;
 	tftp_our_port = WELL_KNOWN_PORT;
+	tftp_windowsize = 1;
+	tftp_next_ack = tftp_windowsize;
 
 #ifdef CONFIG_TFTP_TSIZE
 	tftp_tsize = 0;