Bluetooth: Host: l2cap: Fix MPS/MTU confusion

The implementation used BT_L2CAP_ECRED_MIN_MTU to check the lower limits
of both MTU and MPS, instead of BT_L2CAP_ECRED_MIN_MPS for MPS. While
these are the same here, confusion may arise. This commit fixes the
confusion.

Signed-off-by: Håvard Reierstad <haavard.reierstad@nordicsemi.no>