tc35815: Enable StripCRC feature

The chip can strip CRC automatically on receiving.  Enable it.

Also fix potential RX_BUF_SIZE calculation bug which was obscured by
alignment.  And use proper symbols (NET_IP_ALIGN, ETH_FCS_LEN, etc.)
instead of magic numbers.

Signed-off-by: Atsushi Nemoto <anemo@mba.ocn.ne.jp>
Signed-off-by: David S. Miller <davem@davemloft.net>