ftp read/write sizes

[tridge]

The MissionPlanner ftp code uses read and write sizes of 239, which is the maximum that the protocol supports.
MAVProxy until recently used 110 as SiK radios performed badly with over that size due to radio packet fragmentation.
I just tested RFD900x with MultiPoint firmware 3.0 and found that it can't ftp the parameters at all if you use a burst read size over 80, and for file upload 80 also seems to be the limit
I've opened a PR for MAVProxy to change the defaults to 80:
ArduPilot/MAVProxy#1170
I think we need to test MissionPlanner with RFD900x MultiPoint (we can do that at the conference) and see if we should also change MissionPlanner defaults to 80
It makes a surprisingly small difference in download speed