staging: comedi: me4000: fix the interrupt request/free

Only set the dev->irq when the request_irq is successful.
Use the dev->board_name for the name passed to request_irq.
Change the printk messages into dev_warn().
Make sure free_irq is called in the detach.

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