cmd: mmc: Invalidate MMC block cache after init

Make sure the block cache is cleared for the MMC device after it was
reinitialized to avoid having any stale data in the cache, like e.g.
partition tables or such.

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Cc: Jaehoon Chung <jh80.chung@samsung.com>