mmc: Use proper IS_ENABLED macro to check block support

mmc: Use proper IS_ENABLED macro to check block support

Use CONFIG_IS_ENABLED(BLK) instead of CONFIG_BLK,
in order to fix the following build issues when
CONFIG_SPL_MMC_WRITE is selected:

drivers/mmc/mmc_write.c:69:7: error: conflicting types for 'mmc_berase'
 ulong mmc_berase(struct udevice *dev, lbaint_t start, lbaint_t blkcnt)
       ^~~~~~~~~~
In file included from drivers/mmc/mmc_write.c:15:0:
drivers/mmc/mmc_private.h:39:7: note: previous declaration of 'mmc_berase' was here
 ulong mmc_berase(struct blk_desc *block_dev, lbaint_t start, lbaint_t blkcnt);
       ^~~~~~~~~~
drivers/mmc/mmc_write.c:187:7: error: conflicting types for 'mmc_bwrite'
 ulong mmc_bwrite(struct udevice *dev, lbaint_t start, lbaint_t blkcnt,
       ^~~~~~~~~~
In file included from drivers/mmc/mmc_write.c:15:0:
drivers/mmc/mmc_private.h:37:7: note: previous declaration of 'mmc_bwrite' was here
 ulong mmc_bwrite(struct blk_desc *block_dev, lbaint_t start, lbaint_t blkcnt,
       ^~~~~~~~~~

Signed-off-by: Ezequiel Garcia <ezequiel@collabora.com>
Reviewed-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>