spi: soft_spi: Support NULL din/dout buffers

This mirrors the conventions used in other SPI drivers (kirkwood,
davinci, atmel, et al) where the din/dout buffer can be NULL when the
received/transmitted data isn't important.  This reduces the need for
allocating additional buffers when write-only/read-only functionality is
needed.

In the din == NULL case, the received data is simply not stored.  In the
dout == NULL case, zeroes are transmitted.

Signed-off-by: Andrew Ruder <andrew.ruder@elecsyscorp.com>
Cc: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
Reviewed-by: Jagannadha Sutradharudu Teki <jaganna@xilinx.com>

drivers/spi/soft_spi.c

@@ -136,10 +136,14 @@ int  spi_xfer(struct spi_slave *slave, unsigned int bitlen,
 		/*
 		 * Check if it is time to work on a new byte.
 		 */
-		if((j % 8) == 0) {
-			tmpdout = *txd++;
+		if ((j % 8) == 0) {
+			if (txd)
+				tmpdout = *txd++;
+			else
+				tmpdout = 0;
 			if(j != 0) {
-				*rxd++ = tmpdin;
+				if (rxd)
+					*rxd++ = tmpdin;
 			}
 			tmpdin  = 0;
 		}
@@ -164,9 +168,11 @@ int  spi_xfer(struct spi_slave *slave, unsigned int bitlen,
 	 * bits over to left-justify them.  Then store the last byte
 	 * read in.
 	 */
-	if((bitlen % 8) != 0)
-		tmpdin <<= 8 - (bitlen % 8);
-	*rxd++ = tmpdin;
+	if (rxd) {
+		if ((bitlen % 8) != 0)
+			tmpdin <<= 8 - (bitlen % 8);
+		*rxd++ = tmpdin;
+	}
 
 	if (flags & SPI_XFER_END)
 		spi_cs_deactivate(slave);