iio: dac: Kconfig: Fix build error for ltc2664

If REGMAP_SPI is n and LTC2664 is y, the following build error occurs:

	riscv64-unknown-linux-gnu-ld: drivers/iio/dac/ltc2664.o: in function `ltc2664_probe':
	ltc2664.c:(.text+0x714): undefined reference to `__devm_regmap_init_spi'

Select REGMAP_SPI instead of REGMAP for LTC2664 to fix it.

Fixes: 4cc2fc4 ("iio: dac: ltc2664: Add driver for LTC2664 and LTC2672")
Reviewed-by: Nuno Sa <nuno.sa@analog.com>
Signed-off-by: Jinjie Ruan <ruanjinjie@huawei.com>
Link: https://patch.msgid.link/20241024015553.1111253-1-ruanjinjie@huawei.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

Author: Jinjie Ruan

drivers/iio/dac/Kconfig

@@ -380,7 +380,7 @@ config LTC2632
 config LTC2664
 	tristate "Analog Devices LTC2664 and LTC2672 DAC SPI driver"
 	depends on SPI
-	select REGMAP
+	select REGMAP_SPI
 	help
 	  Say yes here to build support for Analog Devices
 	  LTC2664 and LTC2672 converters (DAC).