powerpc/powernv: Fix opal_exit tracepoint opcode

powerpc/powernv: Fix opal_exit tracepoint opcode

Currently the opal_exit tracepoint usually shows the opcode as 0:

  <idle>-0     [047] d.h.   635.654292: opal_entry: opcode=63
  <idle>-0     [047] d.h.   635.654296: opal_exit: opcode=0 retval=0
  kopald-1209  [019] d...   636.420943: opal_entry: opcode=10
  kopald-1209  [019] d...   636.420959: opal_exit: opcode=0 retval=0

This is because we incorrectly load the opcode into r0 before calling
__trace_opal_exit(), whereas it expects the opcode in r3 (first function
parameter). In fact we are leaving the retval in r3, so opcode and
retval will always show the same value.

Instead load the opcode into r3, resulting in:

  <idle>-0     [040] d.h.   636.618625: opal_entry: opcode=63
  <idle>-0     [040] d.h.   636.618627: opal_exit: opcode=63 retval=0

Fixes: f1742f50589c ("powernv: Add OPAL tracepoints")
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>