net: ethernet: dec: tulip: Fix length mask in receive length calculation

The receive frame length calculation uses a wrong mask to calculate the
length of the received frames.

Per spec table 4-1 the length is contained in the FL (Frame Length)
field in bits 30:16.

This didn't show up as an issue so far since frames were limited to
1500 bytes which falls within the 11 bit window.

Signed-off-by: Moritz Fischer <mdf@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>