Bluetooth: SMP: Fix registering L2CAP fixed channel

After last refactor L2CAP channel context is type of struct
bt_l2cap_le_chan. This fix not being able to use Security Manager.

Change-Id: I6e54a1ab1bc3c48f856b07cb45278e3b8b23b122
Signed-off-by: Szymon Janc <ext.szymon.janc@tieto.com>