mmc: warn about voltage mismatches

Get rid of a silent failure mode when the MMC/SD host doesn't
support the voltages needed to operate a given card, by
adding a warning.  A 3.3V host and a 3.0V card, for example,
no longer need to mysteriously just not work at all.

This isn't the best diagnostic; ideally it would also tell
what voltage the card and host support (and not just by
dumping the bitmasks).
Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Pierre Ossman <drzeus@drzeus.cx>

drivers/mmc/core/core.c

@@ -542,6 +542,8 @@ u32 mmc_select_voltage(struct mmc_host *host, u32 ocr)
 		host->ios.vdd = bit;
 		mmc_set_ios(host);
 	} else {
+		pr_warning("%s: host doesn't support card's voltages\n",
+				mmc_hostname(host));
 		ocr = 0;
 	}