mmc: zynq: Fix tuning_loop_counter type in arasan_sdhci_execute_tuning()

Code around tuning_loop_counter variable expects to go below zero.
That's why this variable can't use unsigned type.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>

drivers/mmc/zynq_sdhci.c

@@ -92,7 +92,7 @@ static int arasan_sdhci_execute_tuning(struct mmc *mmc, u8 opcode)
 	u32 ctrl;
 	struct sdhci_host *host;
 	struct arasan_sdhci_priv *priv = dev_get_priv(mmc->dev);
-	u8 tuning_loop_counter = SDHCI_TUNING_LOOP_COUNT;
+	char tuning_loop_counter = SDHCI_TUNING_LOOP_COUNT;
 	u8 deviceid;
 
 	debug("%s\n", __func__);