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>

drivers/net/caif/Kconfig

@@ -15,7 +15,7 @@ config CAIF_TTY
 
 config CAIF_SPI_SLAVE
 	tristate "CAIF SPI transport driver for slave interface"
-	depends on CAIF
+	depends on CAIF && HAS_DMA
 	default n
 	---help---
 	The CAIF Link layer SPI Protocol driver for Slave SPI interface.