Bluetooth: L2CAP: Fix missing key size check for L2CAP_LE_CONN_REQ

This adds a check for encryption key size upon receiving
L2CAP_LE_CONN_REQ which is required by L2CAP/LE/CFC/BV-15-C which
expects L2CAP_CR_LE_BAD_KEY_SIZE.

Link: https://lore.kernel.org/linux-bluetooth/5782243.rdbgypaU67@n9w6sw14/
Fixes: 27e2d4c8d28b ("Bluetooth: Add basic LE L2CAP connect request receiving support")
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>
Tested-by: Christian Eggers <ceggers@arri.de>