[POWERPC] add check for initialized driver data to pmi driver

This patch adds a check for the private driver data to be initialized.
The bug showed up, as the caller found a pmi device by it's type.
Whereas the pmi driver probes for the type and the name.
Since the name was not as the driver expected, it did not initialize.
A more relaxed probing will be supplied with an extra patch, too.

Signed-off-by: Christian Krafft <krafft@de.ibm.com>
Signed-off-by: Arnd Bergmann <arnd.bergmann@de.ibm.com>