serial: SERIAL_STM32 should depend on HAS_DMA

If NO_DMA=y:

    drivers/built-in.o: In function `stm32_serial_remove':
    stm32-usart.c:(.text+0xcea1a): undefined reference to `bad_dma_ops'
    stm32-usart.c:(.text+0xcea7a): undefined reference to `bad_dma_ops'

Add a dependency on HAS_DMA to fix this.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Alexandre Torgue <alexandre.torgue@st.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>