mmc: sdhci: fix NULL pointer access when host->ops is not set

Until recently, sdhci_ops was used only for overriding IO accessors.
(so, host->ops was not set by any drivers except bcm2835_sdhci.c)

Now, we have more optional callbacks, get_cd, set_control_reg, and
set_clock.  However, the code

    if (host->ops->get_cd)
            host->ops->get_cd(host);

... expects host->ops is set for all drivers.

Commit 5e96217f0434 ("mmc: pic32_sdhci: move the code to
pic32_sdhci.c") and commit 62226b68631b ("mmc: sdhci: move the
callback function into sdhci_ops") added sdhci_ops for pic32_sdhci.c
and s5p_sdhci.c, but the other drivers still do not (need not) set
host->ops because all callbacks in sdhci_ops are optional.

host->ops must be checked to avoid the system crash caused by NULL
pointer access.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

drivers/mmc/sdhci.c

@@ -359,7 +359,7 @@ static int sdhci_set_clock(struct mmc *mmc, unsigned int clock)
 		div >>= 1;
 	}
 
-	if (host->ops->set_clock)
+	if (host->ops && host->ops->set_clock)
 		host->ops->set_clock(host, div);
 
 	clk |= (div & SDHCI_DIV_MASK) << SDHCI_DIVIDER_SHIFT;
@@ -427,7 +427,7 @@ static int sdhci_set_ios(struct mmc *mmc)
 	u32 ctrl;
 	struct sdhci_host *host = mmc->priv;
 
-	if (host->ops->set_control_reg)
+	if (host->ops && host->ops->set_control_reg)
 		host->ops->set_control_reg(host);
 
 	if (mmc->clock != host->clock)
@@ -480,7 +480,7 @@ static int sdhci_init(struct mmc *mmc)
 
 	sdhci_set_power(host, fls(mmc->cfg->voltages) - 1);
 
-	if (host->ops->get_cd)
+	if (host->ops && host->ops->get_cd)
 		host->ops->get_cd(host);
 
 	/* Enable only interrupts served by the SD controller */