spi: bcm2835: fix kbuild compile warnings/errors and a typo

fixes several warnings/error emmitted by the kbuild system:
* warn: cast from pointer to integer of different size
  using size_t instead of u32
* error: 'SZ_4K' undeclared
  moved to PAGE_SIZE and PAGE_MASK instead

Review showed also a typo in the same code where tx_buff
was checked twice instead of checking both rx and tx_buff.

Reported by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Martin Sperl <kernel@martin.sperl.org>
Signed-off-by: Mark Brown <broonie@kernel.org>

drivers/spi/spi-bcm2835.c

@@ -20,6 +20,7 @@
  * GNU General Public License for more details.
  */
 
+#include <asm/page.h>
 #include <linux/clk.h>
 #include <linux/completion.h>
 #include <linux/delay.h>
@@ -378,18 +379,19 @@ static bool bcm2835_spi_can_dma(struct spi_master *master,
 	}
 
 	/* if we run rx/tx_buf with word aligned addresses then we are OK */
-	if (((u32)tfr->tx_buf % 4 == 0) && ((u32)tfr->tx_buf % 4 == 0))
+	if ((((size_t)tfr->rx_buf & 3) == 0) &&
+	    (((size_t)tfr->tx_buf & 3) == 0))
 		return true;
 
 	/* otherwise we only allow transfers within the same page
 	 * to avoid wasting time on dma_mapping when it is not practical
 	 */
-	if (((u32)tfr->tx_buf % SZ_4K) + tfr->len > SZ_4K) {
+	if (((size_t)tfr->tx_buf & PAGE_MASK) + tfr->len > PAGE_SIZE) {
 		dev_warn_once(&spi->dev,
 			      "Unaligned spi tx-transfer bridging page\n");
 		return false;
 	}
-	if (((u32)tfr->rx_buf % SZ_4K) + tfr->len > SZ_4K) {
+	if (((size_t)tfr->rx_buf & PAGE_MASK) + tfr->len > PAGE_SIZE) {
 		dev_warn_once(&spi->dev,
 			      "Unaligned spi tx-transfer bridging page\n");
 		return false;