Make delay values unsigned.

There is no reason for negative delays in our use cases:
 - We don't need it (to work around any quirks).
 - sleep() (POSIX) uses an unsigned argument.
 - usleep() (POSIX) uses an unsigned argument.
 - Sleep() (Windows) uses an unsigned argument.

Change all callees as well (without any complications).

Signed-off-by: Stefan Tauner <stefan.tauner@alumni.tuwien.ac.at>
Acked-by: Stefan Tauner <stefan.tauner@alumni.tuwien.ac.at>

git-svn-id: svn://flashrom.org/flashrom/trunk@1782 2b7e53f0-3cfb-0310-b3e9-8179ed1497e1