leds-mc13892: fix suspend/resume

BugLink: http://bugs.launchpad.net/bugs/537083

The workqueue update for the leds-mc13892 driver introduced a regression
where machine would no longer suspend and resume, as the wrong type was
being retrieved from the platform drvdata.

This change updates the suspend and resume functions to use the struct
mc13892_led type.

Signed-off-by: Jeremy Kerr <jeremy.kerr@canonical.com>
Signed-off-by: Bryan Wu <bryan.wu@canonical.com>
Acked-by: Andy Whitcroft <apw@canonical.com>
Signed-off-by: Andy Whitcroft <apw@canonical.com>