staging: unisys: visorbus: vbuschannel.h: removed VISOR_VBUS_CHANNEL_SIGNATURE

VISOR_VBUS_CHANNEL_SIGNATURE just pointed to VISOR_CHANNEL_SIGNATURE, so
this replaces the redundant definition with VISOR_CHANNEL_SIGNATURE.

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