clk: fix clk_get_rate() documentation

Improve clk_get_rate() @return documentation that otherwise is a bit
ambiguous. At the moment I expect to return 0 as error since the return
type is 'ulong', instead the function really returns negative value in
case the corresponding function pointer is null and returns 0 if the clock
is invalid.

Signed-off-by: Giulio Benetti <giulio.benetti@benettiengineering.com>
Reviewed-by: Sean Anderson <seanga2@gmail.com>