ide: use instance values to hold C drive and channel structures

Rather than iterate over the global IDE channels list, use instance variables to
hold the relevant pointers. This allows us to remove ide_seek_channel() since
it is no longer necessary for each IDE device to iterate over the global list
in order to locate the drive/channel information.

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