staging: comedi: amplc_pci230: use comedi_async 'scans_done' to detect AI EOA

Remove the private data member 'ai_scan_count' and use the comedi_async 'scans_done'
member to detect the analog input end-of-acquisition.

Use the comedi_nsamples_left() helper to work out the number of 'wake' samples in
pci230_ai_update_fifo_trigger_level() and the number of 'todo' samples actually
added to the async buffer in pci230_handle_ai().

Remove the unnecessary COMEDI_CB_OVERFLOW event for the hardware FIFO overflow
error. The COMEDI_CB_ERROR event will terminate the command.

comedi_buf_write_samples() can fail if the async buffer does not have room for
the sample. The it will set the COMEDI_CB_OVERFLOW event and return 0. Detect
this and quit trying to read and add more samples. The event will terminate the
command.

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>