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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).

Corresponding to flashrom svn r1782.
Signed-off-by: Stefan Tauner <stefan.tauner@alumni.tuwien.ac.at>
Acked-by: Stefan Tauner <stefan.tauner@alumni.tuwien.ac.at>