RISC-V: avoid redundant and misleading/wrong error messages

RISC-V: avoid redundant and misleading/wrong error messages

The use of a wrong (for the insn) relocation operator (or a future one
which simply isn't recognized by older gas yet) doesn't render the (rest
of the) expression "bad". Furthermore alongside the error from
expression() in most cases the parser would emit another error then
anyway. Suppress the call to my_getExpression() in such a case,
arranging for a guaranteed subsequent error message by marking the
expression "illegal".