ide: locate drives by iterating over the device tree during ob_ide_quiesce()

It is now possible to locate IDE drives by searching the device tree for "block"
type devices containing a C drive instance variable. Rewrite ob_ide_quiesce() to
locate and quiesce IDE drives using this method which finally enables us to
remove the global IDE channels list and its remaining ide_add_channel() function.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>