ll_temac: Add support for V2 LLTEMAC core

LLTEMAC V1 cores place only received packet length in the app4 word.
V2 cores place additional information in app4.
Mask out the additional information when retrieving the packet length.

Signed-off-by: Steven J. Magnani <steve@digidescorp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>