iio: adc: TI_AM335X_ADC should depend on HAS_DMA

If NO_DMA=y:

    ERROR: "bad_dma_ops" [drivers/iio/adc/ti_am335x_adc.ko] undefined!

Add a dependency on HAS_DMA to fix this.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Fixes: f438b9da (" drivers: iio: ti_am335x_adc: add dma support")
Cc: <stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>