powerpc/fsl-lbc: Fix for fsl_upm

If Freescale LBC driver fails to initialise itself from device tree, then
internal structure is freed only but not NULL-fied.  As result functions
fsl_lbc_find() after checking the structure is not NULL are trying to
access device registers.

Signed-off-by: Alexandre Rusev <arusev@dev.rtsoft.ru>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>