net: macb: check clk_set_rate return value to be negative

clk_set_rate() returns the set rate in case of success and a
negative number in case of failure. Consider failure only the
negative numbers.

Fixes: 3ef64444 ("dm: net: macb: Implement link speed change callback")
Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>