net: ftmac100: set max_mtu to allow DSA overhead setting

net: ftmac100: set max_mtu to allow DSA overhead setting

In case ftmac100 is used with a DSA switch, Linux wants to set MTU
to 1504 to accommodate for DSA overhead. With the default max_mtu
it leads to the error message:
 ftmac100 92000000.mac eth0: error -22 setting MTU to 1504 to include DSA overhead

ftmac100 supports packet length 1518 (MAX_PKT_SIZE constant), so it is
safe to report it in max_mtu.

Signed-off-by: Sergei Antonov <saproj@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://lore.kernel.org/r/20220821160844.474277-1-saproj@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>