staging: vt6656: replaced MAX_PACKET_LEN with ETH_FRAME_LEN

Replaced custom macro with one from include/linux/if_ether.h.
Also removed MAX_LOOKAHEAD_SIZE as it is not used elsewhere.

Signed-off-by: Andres More <more.andres@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>