net: stmmac: xgmac2: disable RBUE in default RX interrupt mask

In the Linux kernel, the following vulnerability has been resolved:

net: stmmac: xgmac2: disable RBUE in default RX interrupt mask

Enabling the RX Buffer Unavailable (RBUE) interrupt is counterproductive
and can trigger a MAC interrupt storm under heavy RX pressure. When the
DMA runs out of RX descriptors it fires RBUE continuously until software
refills the ring.

However, RBUE is redundant: the normal RX completion interrupt (RIE)
already triggers NAPI, which processes completed descriptors and refills
the ring, causing the DMA to resume. The RBUE handler itself only sets
handle_rx - the same outcome as RIE.

On Agilex5 under heavy RX pressure, the MAC interrupt (which includes
RBUE) was observed firing 1,821,811,555 times against only 2,618,627
actual RX completions - a ~695x ratio - confirming the severity of the
storm.

RBUE does not provide OOM recovery. If page_pool is exhausted,
stmmac_rx_refill() cannot advance the DMA tail pointer, the DMA stays
suspended, and RBUE fires again on the next NAPI completion - a storm
with no forward progress. This patch trades that storm for a clean
stall with the same RX outcome. Proper OOM recovery is a pre-existing
gap outside the scope of this fix.

Note: as a consequence of disabling RBUE, the rx_buf_unav_irq ethtool
counter will always read 0 on XGMAC2 devices. This behaviour is already
inconsistent across DWMAC core versions.

Remove RBUE from XGMAC_DMA_INT_DEFAULT_EN and XGMAC_DMA_INT_DEFAULT_RX
to prevent the interrupt storm while keeping normal RX handling intact.