net: systemport: set dev->max_mtu to UMAC_MAX_MTU_SIZE

The driver is already allocating receive buffers of 2KiB and the
Ethernet MAC is configured to accept frames up to UMAC_MAX_MTU_SIZE.

Fixes: bfcb8132 ("net: dsa: configure the MTU for switch ports")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Link: https://lore.kernel.org/r/20201218173843.141046-1-f.fainelli@gmail.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>