clk: at91: Fix use of unsigned loop index

clk: at91: Fix use of unsigned loop index

The use of the unsigned variable 'i' as a loop index leads to the test
for i being non-negative always being true. Instead declare 'i' as an
int so that the for loop will terminate as expected.
If the original for loop completes 'i' will be 1 past the end of the
array so decrement it in the subsequent error path to prevent an out of
bounds access occurring.

This issue was found by Smatch.

Signed-off-by: Andrew Goodbody <andrew.goodbody@linaro.org>