staging: unisys: Lock visorchannels associated with devices

A visorchannel associated with a device should have its writing to
the channel protected by a lock.

Fixes: b32c4997 ('staging: unisys: Move channel creation up the stack')
Signed-off-by: Tim Sell <Timothy.Sell@unisys.com>
Signed-off-by: Benjamin Romer <benjamin.romer@unisys.com>
Signed-off-by: David Kershner <david.kershner@unisys.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>