staging: unisys: Update MAINTAINERS file

Benjamin Romer is no longer a maintainer for the s-Par drivers. David
Kershner will now be the maintainer.

Signed-off-by: David Kershner <david.kershner@unisys.com>
Signed-off-by: Benjamin Romer <benjamin.romer@unisys.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>