usb_disk: returns B_DEV_NOT_READY for SCSI_SENSE_KEY_NOT_READY

...instead of B_DEV_NO_MEDIA. The latter seems to imply we stop testing
the unit is ready in usb_disk_device_added().
Based on a patch provided by markh in #9589.
ntfs: make sure our confidence is higher then the intel partition add-on

This as the intel partition addon just does a very weak test, and the
NTFS test is much safer. This prevents NTFS filesystems that have a
valid boot sector signature but no partition table, from being
picked up by the intel partition table add-on instead of the ntfs
add-on.

Patch provided by markh, thanks!