rss2i.py: Fit the RSS Input Map behaviour to the updated docs

@1559402 Could you please update the default configuration for the RSS Input Map according to the updated docs (see doc#1 (closed) attachment).

Please note, that any RSS Input Map configuration line should revert this default configuration. Hence, the user has to decide whether to specify the vmem config for all frames or to keep the default configuration. This behaviour is pretty much the same as that one for the Sync Map.

E.g. when you have a config file test.conf as follows

frame 0: dev 1 @ 2444 MHz
frame 1: dev 2 @ 2444 MHz
in: frame 0 -> vmem 0

and parse it using

rss2i -a parse -f test.conf -d 1

it should give you an error like e.g.

rss2i: parse: ERROR: There wasn't any vmem assigned to frame 1.