The different chips may have different requests when changing mtu.
Therefore, add a new help function of rtl_ops to change mtu. Besides,
reset the tx/rx after changing mtu.

Additionally, add mtu_to_size() and size_to_mtu() macros to simplify
the code.
Signed-off-by: Hayes Wang <hayeswang@realtek.com>
Signed-off-by: David S. Miller <davem@davemloft.net>