media: venus: fix loop wrap in cleanup of clks

The current pre-decrement is incorrect and should be replaced
with a post-decrement. Consider the case where the very first
clk_prepare_enable fails when i is 0; in this case the error
clean up will decrement the unsigned int which wraps to the
largest unsigned int value causing an array out of bounds read
on core->clks[i].

Detected by CoverityScan, CID#1446590 ("Out-of-bounds read")

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Hans Verkuil <hansverk@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>