staging: unisys: visorbus: fix sloppy unsigned/signed int conversions

Several implicit unsigned --> signed int conversions were corrected.
No runtime effects were observed here, nevertheless implicit conversions
between integral types can lead to unexpected behavior.

Signed-off-by: Tim Sell <Timothy.Sell@unisys.com>
Signed-off-by: David Kershner <david.kershner@unisys.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>