mfd: Improve diagnostics for WM8350 ID register probe

Check the return value of the device I/O functions when reading the
ID registers so we can provide a more useful diagnostic when we're
having trouble talking to the device.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Samuel Ortiz <sameo@openedhand.com>