staging: comedi: me4000: only enable PLX interrupt if we have and irq

Currently me4000_reset() always enables the PLX interrupt. Move the
enable of the interrupt into me4000_auto_attach() and only do the
enable if we actually have and irq.

Make sure the PLX interrupt is disabled in me4000_reset() before we
try to request the irq.

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>