staging: unisys: Use size of channel defined in the channel.

The size of the channel should be pulled from the channel header, not
from the message. All channels must be at least the size of the
channel_header.

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>