staging: comedi: pcmmio: simplify pcmmio_stop_intr()

This function is only called by the interrupt subdevice so the sanity
check of the 'asic' is not necessary. Remove it.

The 'nports' is always 3 and the 'firstport' is always 0. Remove the
for () loop that clears the registers to disable the interrupts and
just use the pcmmio_dio_write() helper to write to the three page
registers.

This also fixes a bug where the write to the page registers is not
protected with the spinlock.

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>