clk: at91: peripheral: fix return value

determine_rate() is expected to return an error code, or 0 on success.
clk_sam9x5_peripheral_determine_rate() has a branch that returns the
parent rate on a certain case. This is the behavior of round_rate(),
so let's go ahead and fix this by setting req->rate.

Fixes: b4c115c7 ("clk: at91: clk-peripheral: add support for changeable parent rate")
Reviewed-by: Alexander Sverdlin <alexander.sverdlin@gmail.com>
Acked-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Signed-off-by: Brian Masney <bmasney@redhat.com>