eth: fbnic: Set AW_FLUSH_MODE alongside AW_FLUSH when flushing the mailbox

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

eth: fbnic: Set AW_FLUSH_MODE alongside AW_FLUSH when flushing the mailbox

When tearing down the FW mailbox Rx ring, fbnic_mbx_reset_desc_ring()
writes AW_CFG with FLUSH set and everything else, BME included, cleared.
Clearing BME halts the device's writes to the host but leaves the staged
requests parked in the PUL write pipeline rather than draining them, so
on the write path FLUSH alone never terminates the outstanding requests
and the flush the firmware waits on never completes.

Add the FLUSH_MODE definition and set both bits so the staged writes
drain out of the pipeline on their own. BME stays cleared, so nothing
lands on the host; it is restored later in fbnic_mbx_init_desc_ring()
when the ring is rebuilt, once the outstanding writes are gone.

The read path is unaffected. AR_CFG has no equivalent mode bit and
AR_FLUSH terminates the outstanding reads by itself, so it is left as
is.

Both writes remain plain stores rather than read-modify-writes. That is
deliberate: the matching write in fbnic_mbx_init_desc_ring() restores
BME and the TLP attributes, and clears both flush bits as a side effect.