Field `min_rate` in `cdio_dvd_struct_t` should be named `max_rate`

[skr4n]

Description

The MMC spec has a command READ DVD STRUCTURE containing a "Maximum rate" in its response.

libcdio's mmc_get_dvd_struct_physical_private(), which handles this command, incorrectly interprets it as a "minimum rate" by extracting it into a field named min_rate.

MMC-6 spec: https://www.13thmonkey.org/documentation/SCSI/mmc6r02g.pdf
(the field is the same even in MMC-3)

[skr4n]

Since cdio_dvd_layer_t which defines the min_rate field is public, this would need to be deferred for a major release of libcdio.