clk: at91: optimize clk_round_rate() for AUDIO_PLL

Stop the search for parent rate when exact match is found.

This makes for 3 clk_round_rate() calls instead of 64 of them on
SAMA5D2-based board when searching for 12.288MHz clock.

Signed-off-by: Michał Mirosław <mirq-linux@rere.qmqm.pl>
Reviewed-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>