mmc: bring back partition init for non-DM MMC drivers

Commit d0851c89

 ("blk: Call part_init() in the post_probe() method")
removed the call to part_init() in mmc.c, as this is done by the DM_MMC
framework.
However Allwinner is (still) relying on a non-DM MMC driver, so we are
now missing the implicit partition init, leading to failing MMC accesses
due to the missing partition information.

Bring the call back just for non-DM MMC driver to fix this regression.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Tested-by: Soeren Moch <smoch@web.de>