spl: mmc: fix switch statement

If CONFIG_SPL_LIBCOMMON_SUPPORT is not defined there is a lone case statement
at the end of the switch leading to a compile error.
Remove the offending case statement.

| common/spl/spl_mmc.c:339:7: error: label at end of compound statement

Signed-off-by: Max Krummenacher <max.krummenacher@toradex.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Acked-by: Marek Vasut <marex@denx.de>