iio: dac: Kconfig: Fix build error for ltc2664

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 <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>