[Seq] Enforce non-negative preset values in FirRegOp printer/parser

[fzi-hielscher]

Parsing a negative preset value for the FirRegOp will currently produce the slightly inappropriate message "preset value too large". On the other hand, the printer does output negative integers if the value's MSB is set.

This PR adds a more specific error message to the parser and ensures the printed value is always non-negative.