[autoboot] Fix shell banner timeout

Commit 5e1fa5c ("[parseopt] Add parse_timeout()") introduced a
regression causing the shell banner timeout value (calculated in
milliseconds) to be treated as a timer tick count, resulting in a
timeout of approximately two minutes rather than the intended two
seconds.

Reported-by: Christian Hesse <list@eworm.de>
Signed-off-by: Michael Brown <mcb30@ipxe.org>