scsi/006: skip cache types which disable read cache for SATA drives

scsi/006: skip cache types which disable read cache for SATA drives

The test case scsi/006 sets four cache types to test target SCSI
devices. Two cache types out of the four, "none" and "write back, no
read (daft)" disable read cache. However, these two types do not work
for SATA drives since SAT specification requires Disable Read Cache is
always set to zero in the caching mode page. It results in invalid
argument error and the test case failure.

To avoid the failure, skip the cache types which disable read cache if
the test devices are SATA drives. To check the device, add a helper
function _test_dev_is_sata in scsi/rc.

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Shin'ichiro Kawasaki <shinichiro.kawasaki@wdc.com>