iio: dac: Fix build error when CONFIG_SPI_MASTER=y && CONFIG_I2C=m

This patch fixes below build error when CONFIG_SPI_MASTER=y && CONFIG_I2C=m:

drivers/built-in.o: In function `ad5064_i2c_write':
drivers/iio/dac/ad5064.c:608: undefined reference to `i2c_master_send'
drivers/built-in.o: In function `ad5064_i2c_register_driver':
drivers/iio/dac/ad5064.c:646: undefined reference to `i2c_register_driver'
drivers/built-in.o: In function `ad5064_i2c_unregister_driver':
drivers/iio/dac/ad5064.c:651: undefined reference to `i2c_del_driver'
make: *** [vmlinux] Error 1

When CONFIG_I2C=m, meaning we can't build the drivers in with I2C support.
Thus don't allow the drivers to be compiled as built-in when CONFIG_I2C=m.

The real fix though is to break the driver apart into a SPI part, an I2C part
and a common part. But that's something for 3.11 while this is something for
3.10/stable.

Reported-by: Wu Fengguang <fengguang.wu@intel.com>
Reported-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Axel Lin <axel.lin@ingics.com>
Acked-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>