Bluetooth: L2CAP: Fix using s16_t to represent credits

Credits are 2 octects long so an s16_t positive portion can only half to
the theorical maximum number of credits, so instead this uses u16_t and
do a bound check instead of checking for negative values.

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