staging: comedi: keep the consistency

Changed "register 0x%x" to "register=0x%x" to keep the consistency
of this file.

Signed-off-by: Geliang Tang <geliangtang@163.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>