Bluetooth: L2CAP: Remove use of k_sem for credits

With the changes that introduced a queue k_sem is only used with
K_NO_WAIT which means it is no longer possible to wait/block for credits
so the usage of k_sem is no longer needed and can be safely replaced
with atomic_t just to count the available credits at a given instant.

Fixes #19922

Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>