stack::l2cap Properly type tL2CAP_LE_RESULT_CODE

Value in spec is 16 bits.

Version 5.4 | Vol 3, Part A 4.26 L2CAP_CREDIT_BASED_CONNECTION_RSP (CODE 0x18) Table 4.17

Bug: 350490212
Test: m .
Flag: EXEMPT, Mechanical Refactor

Change-Id: I58ba01cfb5d9e115ec1737d64a5d5b357d1b0b7b