iio: dac: adi-axi-dac: fix wrong register bitfield

iio: dac: adi-axi-dac: fix wrong register bitfield

Fix ADI_DAC_R1_MODE of AXI_DAC_REG_CNTRL_2.

Both generic DAC and ad3552r DAC IPs docs are reporting
bit 5 for it.

Link: https://wiki.analog.com/resources/fpga/docs/axi_dac_ip
Fixes: 4e3949a192e4 ("iio: dac: add support for AXI DAC IP core")
Cc: stable@vger.kernel.org
Signed-off-by: Angelo Dureghello <adureghello@baylibre.com>
Reviewed-by: Nuno Sa <nuno.sa@analog.com>
Link: https://patch.msgid.link/20241008-wip-bl-ad3552r-axi-v0-iio-testing-v5-1-3d410944a63d@baylibre.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>