iio: ad5064: Move bus write callbacks to #if protected sections

Move the SPI and I2C specific write callbacks to the respective
"#if IS_ENABLED(CONFIG_SPI_MASTER)" and "#if IS_ENABLED(CONFIG_I2C)"
protected sections of the code.

This fixes the following warning which occurs if CONFIG_I2C is not set:
	drivers/iio/dac/ad5064.c: In function ‘ad5064_i2c_write’:
	drivers/iio/dac/ad5064.c:132: error: implicit declaration of function ‘i2c_master_send’

And the follwing warning which occurs when CONFIG_SPI_MASTER is not set:
	drivers/iio/dac/ad5064.c:137: warning: ‘ad5064_spi_write’ defined but not used

Reported-by: Randy Dunlap <rdunlap@xenotime.net>
Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>