staging: comedi: drivers: dev->board_name is always valid

The dev->board_name is always initialized before calling the(*attach)
or (*auto_attach) function. The "BUG" check in comedi_device_postconfig()
is no longer necessary.

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