iio: dac: fix ad9739a random config compile error

WARNING: unmet direct dependencies detected for REGMAP_SPI
  Depends on [n]: SPI [=n]
  Selected by [m]:
  - AD9739A [=m] && IIO [=m] && (SPI [=n] || COMPILE_TEST [=y])
...
ERROR: modpost: "spi_write_then_read" [drivers/base/regmap/regmap-spi.ko] undefined!
ERROR: modpost: "spi_async" [drivers/base/regmap/regmap-spi.ko] undefined!
ERROR: modpost: "spi_sync" [drivers/base/regmap/regmap-spi.ko] undefined!
ERROR: modpost: "__spi_register_driver" [drivers/iio/dac/ad9739a.ko] undefined!

Fixes: e77603d5

 ("iio: dac: support the ad9739a RF DAC")
Reported-by: k2ci <kernel-bot@kylinos.cn>
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202404250156.2PQRWmex-lkp@intel.com/
Signed-off-by: Ke Sun <sunke@kylinos.cn>
Reviewed-by: Nuno Sa <nuno.sa@analog.com>
Link: https://lore.kernel.org/r/20240609030847.2869455-1-sunke@kylinos.cn
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>