staging: comedi: pcl816: clarify irq request in pcl816_attach()

All the board types can use IRQ 2-7 for async command support. Remove
the 'IRQbits', which is a mask of the valid IRQs, from the boardinfo
and refactor pcl816_attach().

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>