net: send RFC1542 compliant value for bootp requests

RFC1542, 3.2:
"The 'secs' field of a BOOTREQUEST message SHOULD represent the
elapsed time, in seconds, since the client sent its first BOOTREQUEST
message.  Note that this implies that the 'secs' field of the first
BOOTREQUEST message SHOULD be set to zero."

Signed-off-by: Stefan Brüns <stefan.bruens@rwth-aachen.de>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>