arch: arm: mach-omap2: mmc: Move omap_mmc_notify_cover_event() prototype

When building the kernel with W=1 the build system complains of:

 drivers/mmc/host/omap.c:854:6: warning: no previous prototype for ‘omap_mmc_notify_cover_event’ [-Wmissing-prototypes]
 854 | void omap_mmc_notify_cover_event(struct device *dev, int num, int is_closed)
 | ^~~~~~~~~~~~~~~~~~~~~~~~~~~

If we move the prototype into a shared headerfile the build system
will be satisfied.  Rather than create a whole new headerfile just
for this purpose, it makes sense to use the already existing
mmc-omap.h.

Cc: Ulf Hansson <ulf.hansson@linaro.org>
Cc: linux-mmc@vger.kernel.org
Cc: Tony Lindgren <tony@atomide.com>
Cc: linux-omap@vger.kernel.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Acked-by: Tony Lindgren <tony@atomide.com>
Link: https://lore.kernel.org/r/20200701102317.235032-1-lee.jones@linaro.orgSigned-off-by: Ulf Hansson <ulf.hansson@linaro.org>

arch/arm/mach-omap2/mmc.h

@@ -16,7 +16,3 @@ static inline int omap_msdi_reset(struct omap_hwmod *oh)
 	return 0;
 }
 #endif
-
-/* called from board-specific card detection service routine */
-extern void omap_mmc_notify_cover_event(struct device *dev, int slot,
-					int is_closed);

include/linux/platform_data/mmc-omap.h

@@ -116,3 +116,6 @@ struct omap_mmc_platform_data {
 
 	} slots[OMAP_MMC_MAX_SLOTS];
 };
+
+extern void omap_mmc_notify_cover_event(struct device *dev, int slot,
+					int is_closed);