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: 4cc2fc445d2e ("iio: dac: ltc2664: Add driver for LTC2664 and LTC2672")
Reviewed-by: Nuno Sa <[e-mail]>
Signed-off-by: Jinjie Ruan <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jonathan Cameron <[e-mail]>

drivers/iio/dac/Kconfig

···
 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).