net: dsa: b53: fix max MTU for 1g switches

net: dsa: b53: fix max MTU for 1g switches

[ Upstream commit 680a8217dc00dc7e7da57888b3c053289b60eb2b ]

JMS_MAX_SIZE is the ethernet frame length, not the MTU, which is payload
without ethernet headers.

According to the datasheets maximum supported frame length for most
gigabyte swithes is 9720 bytes, so convert that to the expected MTU when
using VLAN tagged frames.

Fixes: 6ae5834b983a ("net: dsa: b53: add MTU configuration support")
Signed-off-by: Jonas Gorski <jonas.gorski@gmail.com>
Reviewed-by: Florian Fainelli <florian.fainelli@broadcom.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>