serial: qcom-geni: do not advance stale DMA completions

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

serial: qcom-geni: do not advance stale DMA completions

The qcom GENI serial DMA TX completion path advances the transmit fifo by
the number of bytes recorded in port->tx_remaining.

If uart_flush_buffer() runs after the hardware has completed a DMA
transfer but before the DMA completion interrupt has been handled, the
serial core resets the transmit fifo while port->tx_remaining still
describes the old DMA transfer.

A previous fix avoided advancing an empty fifo by checking that the fifo
length is at least tx_remaining. That still does not distinguish the old
DMA payload from new bytes written after the flush. If userspace writes
new data before the stale DMA completion interrupt is handled, the fifo
can again contain at least tx_remaining bytes and the stale completion
can advance and discard those new bytes.

Mark an in-flight DMA transfer stale when the transmit fifo is flushed.
The later completion still unprepares the original DMA mapping using the
saved length, but it no longer advances the transmit fifo.