Bluetooth/tester: fix coverity issue in l2cap

i is of uint type, so it'll never be negative. Thus, while loop
condition i >= 0 will always be true.
This patch also fixes issue with marking unused l2cap channels
as free by freeing only these that were marked as in_use by
get_free_channel(), and not all of them.

Signed-off-by: Krzysztof Kopyściński <krzysztof.kopyscinski@codecoup.pl>