fwip(4): fixing kernel panic when receiving unicast packet

Wrapping fwip_unicast_input() with NET_EPOCH_{ENTER,EXIT} to avoid a
NET_EPOCH_ASSERT() in netisr_dispatch().

Reviewed by: hselasky
MFC after: 2 weeks

(cherry picked from commit bdf316e892e9a7afa70e094135b5d05fbc26e867)