MMC: Do not use mmc_debug if CONFIG_MMC_BCM2835 is not set

If CONFIG_MMC_BCM2835 was not set the compiling of the kernel failed
since mmc_debug was not defined but used in drivers/mmc/core/quirks.c.

This patch add a ifdef-check for CONFIG_MMC_BCM2835 to the change of
commit 64d39545