net: airoha_eth: increase max MTU to 9220 for DSA jumbo frames

The industry standard jumbo frame MTU is 9216 bytes. When using the DSA
subsystem, a 4-byte tag is added to each Ethernet frame.

Increase AIROHA_MAX_MTU to 9220 bytes (9216 + 4) so that users can set a
standard 9216-byte MTU on DSA ports.

The underlying hardware supports significantly larger frame sizes
(approximately 16K). However, the maximum MTU is limited to 9220 bytes
for now, as this is sufficient to support standard jumbo frames and does
not incur additional memory allocation overhead.

Signed-off-by: Sayantan Nandy <sayantann11@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Acked-by: Lorenzo Bianconi <lorenzo@kernel.org>
Link: https://patch.msgid.link/20260119073658.6216-1-sayantann11@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>