maple: more robust device detection.

Replacement second-in-series patch:

This patch fixes up memory leaks and, by delaying initialisation, makes
device detection more robust.

It also makes clearer the difference between struct maple_device and
struct device, as well as cleaning up the interrupt request code
(without changing its function in any way).

Also now removes redundant registration checking.

Signed-off-by: Adrian McMenamin <adrian@mcmen.demon.co.uk>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>