iio: dac: adi-axi: fix a mistake in axi_dac_ext_info_set()

The last parameter of these axi_dac_(frequency|scale|phase)_set()
functions is supposed to be true for TONE_2 and false for TONE_1. The
bug is the last call where it passes "private - TONE_2". That
subtraction is going to be zero/false for TONE_2 and and -1/true for
TONE_1. Fix the bug, and re-write it as "private == TONE_2" so it's
more obvious what is happening.

Fixes: 4e3949a192e4 ("iio: dac: add support for AXI DAC IP core")
Signed-off-by: Dan Carpenter <[e-mail]>
Reviewed-by: Nuno Sa <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jonathan Cameron <[e-mail]>

drivers/iio/dac/adi-axi-dac.c

···
 	case AXI_DAC_FREQ_TONE_1:
 	case AXI_DAC_FREQ_TONE_2:
 		return axi_dac_frequency_set(st, chan, buf, len,
-					     private - AXI_DAC_FREQ_TONE_1);
+					     private == AXI_DAC_FREQ_TONE_2);
 	case AXI_DAC_SCALE_TONE_1:
 	case AXI_DAC_SCALE_TONE_2:
 		return axi_dac_scale_set(st, chan, buf, len,
-					 private - AXI_DAC_SCALE_TONE_1);
+					 private == AXI_DAC_SCALE_TONE_2);
 	case AXI_DAC_PHASE_TONE_1:
 	case AXI_DAC_PHASE_TONE_2:
 		return axi_dac_phase_set(st, chan, buf, len,
-					 private - AXI_DAC_PHASE_TONE_2);
+					 private == AXI_DAC_PHASE_TONE_2);
 	default:
 		return -EOPNOTSUPP;
 	}