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>