sr: fx2lafw: Abort pending transfers when sampling is stopped

sr: fx2lafw: Abort pending transfers when sampling is stopped

The recent reworks of the fx2lafw made sure that the total buffer size is large
enough hold 500ms of data. This was done to improve performance and stability.
That the timeout value for a transfer was also increased to over 500ms, a side
effect of this is that when sampling is stopped there will be a additional delay
of 500ms. This is because the driver waits for all transfers to be freed
before it sends a SR_DF_END packet. Once sampling has stopped this will only
happen once a transfer times out. This patch cancels all pending transfers when
sampling is stopped, this will cause them to be freed almost immediately and the
additional delay will disappear.

Also make sure, that if we know, that we just have received the last transfer to
not resubmit this transfer again.

Signed-off-by: Lars-Peter Clausen <redacted>