part: call part_init() in blk_get_device_by_str() only for MMC

In blk_get_device_by_str(), the comment says: "Updates the partition table
for the specified hw partition."
Since hw partition is supported only on MMC, it makes no sense to do so
for other devices.

Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
Reviewed-by: Simon Glass <sjg@chromium.org>