i.MX28: Check if WP detection is implemented at all

If the WP function is NULL, simply assume the card is always RW.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Wolfgang Denk <wd@denx.de>
Cc: Detlev Zundel <dzu@denx.de>
Cc: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Stefano Babic <sbabic@denx.de>

drivers/mmc/mxsmmc.c

@@ -133,7 +133,8 @@ mxsmmc_send_cmd(struct mmc *mmc, struct mmc_cmd *cmd, struct mmc_data *data)
 		/* READ or WRITE */
 		if (data->flags & MMC_DATA_READ) {
 			ctrl0 |= SSP_CTRL0_READ;
-		} else if (priv->mmc_is_wp(mmc->block_dev.dev)) {
+		} else if (priv->mmc_is_wp &&
+			priv->mmc_is_wp(mmc->block_dev.dev)) {
 			printf("MMC%d: Can not write a locked card!\n",
 				mmc->block_dev.dev);
 			return UNUSABLE_ERR;