staging: comedi: ni_tio: Use data[insn->n-1] in ni_tio_insn_write()

The `insn_write` handler for the counter subdevices
(`ni_tio_insn_write()`) writes a single data value `data[0]` to the
channel.  Technically, `insn->n` specifies the number of successive
values from `data[]` to write to the channel, but when there is little
benefit in writing multiple data values, the usual Comedi convention is
to just write the last data value `data[insn->n - 1]`.  Change the
function to follow that convention and use `data[insn->n - 1]` instead
of `data[0]`.  (In practice, `insn->n` would normally be 1 anyway.)

Also follow the usual Comedi convention and return `insn->n` from the
handler to indicate success instead of 0 (although any non-negative
return value will do).
Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/comedi/drivers/ni_tio.c

@@ -1682,9 +1682,11 @@ int ni_tio_insn_write(struct comedi_device *dev,
 	unsigned int cidx = counter->counter_index;
 	unsigned int chip = counter->chip_index;
 	unsigned int load_reg;
+	unsigned int load_val;
 
 	if (insn->n < 1)
 		return 0;
+	load_val = data[insn->n - 1];
 	switch (channel) {
 	case 0:
 		/*
@@ -1697,7 +1699,7 @@ int ni_tio_insn_write(struct comedi_device *dev,
 		 * load register is already selected.
 		 */
 		load_reg = ni_tio_next_load_register(counter);
-		ni_tio_write(counter, data[0], load_reg);
+		ni_tio_write(counter, load_val, load_reg);
 		ni_tio_set_bits_transient(counter, NITIO_CMD_REG(cidx),
 					  0, 0, GI_LOAD);
 		/* restore load reg */
@@ -1705,17 +1707,17 @@ int ni_tio_insn_write(struct comedi_device *dev,
 			     load_reg);
 		break;
 	case 1:
-		counter_dev->regs[chip][NITIO_LOADA_REG(cidx)] = data[0];
-		ni_tio_write(counter, data[0], NITIO_LOADA_REG(cidx));
+		counter_dev->regs[chip][NITIO_LOADA_REG(cidx)] = load_val;
+		ni_tio_write(counter, load_val, NITIO_LOADA_REG(cidx));
 		break;
 	case 2:
-		counter_dev->regs[chip][NITIO_LOADB_REG(cidx)] = data[0];
-		ni_tio_write(counter, data[0], NITIO_LOADB_REG(cidx));
+		counter_dev->regs[chip][NITIO_LOADB_REG(cidx)] = load_val;
+		ni_tio_write(counter, load_val, NITIO_LOADB_REG(cidx));
 		break;
 	default:
 		return -EINVAL;
 	}
-	return 0;
+	return insn->n;
 }
 EXPORT_SYMBOL_GPL(ni_tio_insn_write);