staging: unisys: visorbus: replaced vague variable name in zoneguid_show

The variable name "s" doesn't indicate the purpose of the string, which
is to store the id collected from the visorchannel_zoneid function. This
just replaces the name with "zoneid".

Signed-off-by: Alexander Curtin <alexander.curtin@unisys.com>
Signed-off-by: David Kershner <david.kershner@unisys.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>