Staging: vt6655: use ETH_FRAME_LEN macro instead of custom one

Replace custom maximum packet lenght definition MAX_PACKET_LEN by
ETH_FRAME_LEN from <linux/if_ether.h>.

Signed-off-by: Charles Clément <caratorn@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>