net: macb: Shorten max_tx_len to 4KiB - 56 on mpfs

commit ffe467c801347d8b48ab76617e75a492fa038070 from
https://github.com/linux4microchip/linux.git

 linux-6.1-mchp+fpga

On mpfs, with SRAM configured for 4 queues, setting max_tx_len
to GEM_TX_MAX_LEN=0x3f0 results multiple AMBA errors.
Setting max_tx_len to (4KiB - 56) removes those errors.

The details are described in erratum 1686 by Cadence

The max jumbo frame size is also reduced for mpfs to (4KiB - 56).

Signed-off-by: Daire McNamara <daire.mcnamara@microchip.com>
Acked-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Saravanan.K.S <saravanan.kadambathursubramaniyam@windriver.com>