staging: clocking-wizard: Handle invalid clk in notifier

Even though it should never happen, handle the case that the clock
notifier is called with an unexpected clock handle. This avoids the
compiler warning:
   drivers/staging/clocking-wizard/clk-xlnx-clock-wizard.c: In function 'clk_wzrd_clk_notifier':
>> drivers/staging/clocking-wizard/clk-xlnx-clock-wizard.c:99:6: warning: 'max' may be used uninitialized in this function [-Wmaybe-uninitialized]
      if (ndata->new_rate > max)
         ^

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Soren Brinkmann <soren.brinkmann@xilinx.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>