[patch, nios2] correct encoding of "eret" instruction

[Sandra Loosemore]

I've checked in this patch to fix a bug (reported to us by Altera) in 
the encoding of the "eret" instruction in the Nios II assembler and 
disassembler.  Digging around, I saw that the incorrect encoding was 
inherited from Altera's the original version of the binutils port, but 
parroting the error in the test case was due to some later stupidity. 
:-S  Anyway, fixed now.

-Sandra

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