When during tftp receive operation a ACK packet can't successfully be sent
the timer was resetted directly after send resulting in a deadlock.

This patch changes the timer reset operation in a way that it is only resetted
when the actually is progress, namely in the TFTP_ACK/TFTP_DATA pathes in the
tftp handler.

Reported-by: Antony Pavlov <antonynpavlov@gmail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Tested-by: Antony Pavlov <antonynpavlov@gmail.com>