cmd: read: use part_get_info_by_dev_and_name_or_num() instead of open-coded dev_part...

cmd: read: use part_get_info_by_dev_and_name_or_num() instead of open-coded dev_part parsing

Use the helper part_get_info_by_dev_and_name_or_num() for parsing a
dev[:part] string and obtaining the partition info in one go, instead
of open-coding all that.

As a bonus, this will automatically allow using the dev#partname
syntax as well, for accessing raw partitions by name.

Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>