powerpc/fsl: fix compiler warning in fsl_wakeup_sys_init()

Commit c93bd175 ("powerpc/fsl: move to use bus_get_dev_root()")
changed to use bus_get_dev_root() but didn't consider that the function
can fail and return an uninitialized value of ret (hint, the function
can never fail, but the compiler doesn't know that.)

Fix this up by setting ret to -EINVAL just in case something really goes
wrong with the call to bus_get_dev_root().

Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Nicholas Piggin <npiggin@gmail.com>
Cc: Christophe Leroy <christophe.leroy@csgroup.eu>
Cc: linuxppc-dev@lists.ozlabs.org
Fixes: c93bd175

 ("powerpc/fsl: move to use bus_get_dev_root()")
Reported-by: kernel test robot <lkp@intel.com>
Link: https://lore.kernel.org/oe-kbuild-all/202303280045.4oaaezcn-lkp@intel.com/
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Link: https://lore.kernel.org/r/20230327181606.1424846-1-gregkh@linuxfoundation.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>