staging: comedi: mite: introduce mite_ack_linkc()

Introduce a helper function to handle the ack of a LINKC interrupt.
Tidy up the drivers that use the new helper.

The extra check for CHSR_INT in the ni_pcidio driver is not necessary.
This bit will be set if any of the interrupt sources, including CHSR_LINKC,
have generated an interrupt. Remove the extra check.

The mite_get_status() function is now only used by the mite driver.
Make it static and remove the export.

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>