cam: remove redundant scsi_vpd_block_characteristics definition

There were two definitions for the SCSI VPD Block Device Characteristics (page
0xb1): struct scsi_vpd_block_characteristics and struct
scsi_vpd_block_device_characteristics. The latter is more complete and more
widely used. Convert uses of the former to the latter by tweaking the da driver
and removing sturct scsi_vpd_block_characteristics.