staging: comedi: pcmmio: tidy up pcmmio_start_intr()

Refactor the function to remove some indent levels.

Use pcmmio_dio_write() to write the polarity and enable paged registers
instead of using the for () loop to write each register in the pages.

The for () loop actually has a bug. It switches the page to the 'enab'
registers for the first port then switches to the 'pol' registers for
all remaining writes. It also was not using the pagelock spinlock to
protect the writes to the page registers. Using the pcmmio_dio_write()
helper ensures that the writes to the paged registers complete correctly.

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>