TFTP: send legal timeout value

Eric Mertens posted bug #3003705: when we made TFTP use the
correct timeout option when sent to the server (fixed May 18th
2010) it became obvious that libcurl used invalid timeout values
(300 by default while the RFC allows nothing above 255). While of
course it is obvious that as TFTP has worked thus far without
being able to set timeout at all, just removing the setting
wouldn't make any difference in behavior. I decided to still keep
it (but fix the problem) as it now actually allows for easier
(future) customization of the timeout.

(http://curl.haxx.se/bug/view.cgi?id=3003705)