r8169: expand received packet length indication.

8168d and above allow jumbo frames beyond 8k. Bump the received
packet length check before enabling jumbo frames on these chipsets.

Frame length indication covers bits 0..13 of the first Rx descriptor
32 bits for the 8169 and 8168. I only have authoritative documentation
for the allowed use of the extra (13) bit with the 8169 and 8168c.
Realtek's drivers use the same mask for the 816x and the fast ethernet
only 810x.

Signed-off-by: Francois Romieu <romieu@fr.zoreil.com>