net: spacemit: k1-emac: fix jumbo frame support

net: spacemit: k1-emac: fix jumbo frame support

The driver never programs the MAC frame size and jabber registers,
causing the hardware to reject frames larger than the default 1518
bytes even when larger DMA buffers are allocated.

Program MAC_MAXIMUM_FRAME_SIZE, MAC_TRANSMIT_JABBER_SIZE, and
MAC_RECEIVE_JABBER_SIZE based on the configured MTU. Also fix the
maximum buffer size from 4096 to 4095, since the descriptor buffer
size field is only 12 bits. Account for double VLAN tags in frame
size calculations.

Fixes: bfec6d7f2001 ("net: spacemit: Add K1 Ethernet MAC")
Cc: stable@vger.kernel.org
Signed-off-by: Tomas Hlavacek <tmshlvck@gmail.com>
Link: https://patch.msgid.link/20260130102301.477514-1-tmshlvck@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>