staging: unisys: remove wmb() in visordriver_remove_device

Don't need to have a wmb() in visordriver_remove_device. Also removed
an unnecessary check for drv being null.

Signed-off-by: David Kershner <david.kershner@unisys.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>