staging: unisys: visorchannel_clear(): No need to use vmalloc here

Using a page is sufficient, and avoids the cost of vmalloc/vfree

Signed-off-by: Jes Sorensen <Jes.Sorensen@redhat.com>
Signed-off-by: Benjamin Romer <benjamin.romer@unisys.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>