staging: comedi: ni_6527: change driver name string

Change the driver name string used in data structures, kernel messages,
etc. from "ni6527" to "ni_6527" to match the module name.  Use a macro
`DRIVER_NAME` that expands to this string literal.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Reviewed-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>