The UIC result registers (UICCMDARG2/3) are only valid between a
command's completion and the next command's submission. They were read
after uic_cmd_lock was dropped, so a concurrent UIC submitter could
overwrite them in between. Read them into locals before releasing the
lock.

Also mask the generic error code to its [7:0] field when checking it,
so unrelated bits in UICCMDARG2 (such as the attribute set type echoed
for DME_SET) cannot be mistaken for an error.

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