Revert "SMP: Assume L2cap tx completes immediately"

This reverts commit 13b310b3.

The assumption in the original CL "L2cap tx completes immediately"
seems to be incorrect.

Bug: 314819704
Bug: 301168932
Test: m -j
Test: Verify successful pairing with Keyboard Craft repeatedly

Change-Id: I8c13902f5d8c99001fbe1bea417eccf664904f1a