staging: comedi: addi_apci_2032: set COMEDI_CB_OVERFLOW

If the call to `comedi_buf_put()` fails in the interrupt routine, set
the `COMEDI_CB_OVERFLOW` event flag.  Note that the `COMEDI_CB_ERROR`
flag will have also been set by `comedi_buf_put()` in this case.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>