blk: set log2blksz in blk_create_device()

The ext4 file system requires log2blksz to be set. So when setting the
block size on the block descriptor we should fill this field too.

This fixes a problem with EFI block devices providing ext4 partitions, cf.
https://lists.denx.de/pipermail/u-boot/2019-October/387702.html

.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>