staging: unisys: visorbus: Remove pointer cast that causes sparse warning

Removes unnecessary cast of to __iomem of a non-io-mapped pointer, thereby
eliminating the following sparse warning:

visorchannel.c:159:17: warning: cast adds address space to expression (<asn:2>)

Signed-off-by: David Binder <david.binder@unisys.com>
Signed-off-by: David Kershner <david.kershner@unisys.com>
Reviewed-by: Tim Sell <timothy.sell@unisys.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>