net: stmmac: Fixed mtu channged by cache aligned

Since the original mtu is not used when the mtu is updated,
the mtu is aligned with cache, this will get an incorrect.
For example, if you want to configure the mtu to be 1500,
but mtu 1536 is configured in fact.

Fixed: eaf4fac4

 ("net: stmmac: Do not accept invalid MTU values")
Signed-off-by: David Wu <david.wu@rock-chips.com>
Link: https://lore.kernel.org/r/20210113034109.27865-1-david.wu@rock-chips.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>