clk: at91: usb: fix determine_rate prototype

Commit c67881fc is a backport of
0b67c43c upstream commit, fixing a
bug on clk rate change propagation.
But in 4.0 ->determine_rate() prototype has changed, thus introducing
a prototype mismatch when applying it on 3.19.

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Sasha Levin <sasha.levin@oracle.com>