spi: Fix comparison of different integer types

Fix problem discovered with sparse:
+ drivers/spi/spi.c:1554:37: sparse: incompatible types in comparison expression (different signedness)
drivers/spi/spi.c: In function 'spi_write_then_read':
drivers/spi/spi.c:1554:23: warning: comparison of distinct pointer types lacks a cast [enabled by default]

The change to SPI_BUFSIZ was introduced in commit b3a223e, "spi:
Remove SPI_BUFSIZ restriction on spi_write_then_read()"

Cc: Mark Brown <broonie@opensource.wolfsonmicro.com>
Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>

drivers/spi/spi.c

@@ -1551,7 +1551,7 @@ int spi_write_then_read(struct spi_device *spi,
 	 * using the pre-allocated buffer or the transfer is too large.
 	 */
 	if ((n_tx + n_rx) > SPI_BUFSIZ || !mutex_trylock(&lock)) {
-		local_buf = kmalloc(max(SPI_BUFSIZ, n_tx + n_rx), GFP_KERNEL);
+		local_buf = kmalloc(max((unsigned)SPI_BUFSIZ, n_tx + n_rx), GFP_KERNEL);
 		if (!local_buf)
 			return -ENOMEM;
 	} else {