clk: sandbox: don't check clk ID against 0

clk->id is unsigned, so it can't be < 0. Remove the check for that.

FWIW, this issue was introduced when the clock API converted e.g.
clk_get_rate()'s clock ID parameter from an int to an unsigned long
(with a struct clk), without removing this check.

Fixes: 135aa950 ("clk: convert API to match reset/mailbox style")
Reported-by: Coverity Scan
Signed-off-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Simon Glass <sjg@chromium.org>