net: ethernet: stmicro: stmmac: permit MTU change with interface up

Remove the limitation where the interface needs to be down to change
MTU by releasing and opening the stmmac driver to set the new MTU.
Also call the set_filter function to correctly init the port.
This permits to remove the EBUSY error while the ethernet port is
running permitting a correct MTU change if for example a DSA request
a MTU change for a switch CPU port.

Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>