caif-driver: add HAS_DMA dependency

Fix this error on an s390 allyesconfig build:

linux-2.6/drivers/net/caif/caif_spi.c:98:
    undefined reference to `dma_free_coherent'

Cc: Sjur Braendeland <sjur.brandeland@stericsson.com>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>