jqueue: Don't modify input opcode when changing priority

Commit 4679547e implemented the ability to change job's priority after it
was submitted. The code contained a bug whereby it would modify the
input data for an opcode, something the job queue shouldn't do (logical
units do for historical reasons).

This patch removes the line modifying the opcode input and adjusts the
tests.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>