avr32: handle NULL as a valid clock object

Since NULL is used as valid clock object on optional clocks we have to handle
this case in avr32 implementation as well.

Fixes: e1824dfe (net: macb: Adjust tx_clk when link speed changes)
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Hans-Christian Egtvedt <egtvedt@samfundet.no>