ARM: OMAP: Fix build error when mmc_omap is built as module

Otherwise we get the following error:

arch/arm/mach-omap2/built-in.o: In function `n8x0_mmc_callback':
twl-common.c:(.text+0x108a0): undefined reference to
`omap_mmc_notify_cover_event'

Fix this by warning about unusable MMC cover events.

The long term fix needs to change the MMC drivers to
register board specific callbacks directly with PMIC.
Reported-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Tony Lindgren <tony@atomide.com>

arch/arm/mach-omap2/board-n8x0.c

@@ -371,7 +371,11 @@ static void n8x0_mmc_callback(void *data, u8 card_mask)
 	else
 		*openp = 0;
 
+#ifdef CONFIG_MMC_OMAP
 	omap_mmc_notify_cover_event(mmc_device, index, *openp);
+#else
+	pr_warn("MMC: notify cover event not available\n");
+#endif
 }
 
 static int n8x0_mmc_late_init(struct device *dev)