[PATCH] forcedeth: Update error handling

Ayaz wrote an update to the error handling for forcedeth (which I
modified heavily, thus all bugs are mine):
The ERROR4 bit is not a fatal error, it just indicates a mismatch
between the actual packet len and the len according to the 802.3 header.
The patch adds proper handling.
The patch also removes the code that drops all packets with RX_ERROR &
(!RX_FRAMINGERR): ERROR4 errors are also not fatal.