blackfin: bf60x: clock: return 0 upon error from clk_round_rate()

clk_round_rate() should return 0 upon an error, rather than returning
a negative error code.  This is because clk_round_rate() is being
changed to return an unsigned return type rather than a signed type,
since some clock sources can generate rates higher than (2^31)-1 Hz.
Signed-off-by: Paul Walmsley <pwalmsley@nvidia.com>
Signed-off-by: Steven Miao <realmz6@gmail.com>