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

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 <dan.carpenter@linaro.org>
Reviewed-by: Nuno Sa <nuno.sa@analog.com>
Link: https://lore.kernel.org/r/df7c6e1b-b619-40c3-9881-838587ed15d4@moroto.mountain
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>