powerpc: Fix a kernel-doc warning

The current code provokes a kernel-doc warnings:
arch/powerpc/kernel/process.c:1606: warning: This comment starts with
'/**', but isn't a kernel-doc comment. Refer
Documentation/doc-guide/kernel-doc.rst

Signed-off-by: Bo Liu <[e-mail]>
Reviewed-by: Christophe Leroy <[e-mail]>
Signed-off-by: Michael Ellerman <[e-mail]>
Link: https://msgid.link/[e-mail]

arch/powerpc/kernel/process.c

···
 }
 
 #ifdef CONFIG_PPC64
-/**
+/*
  * Assign a TIDR (thread ID) for task @t and set it in the thread
  * structure. For now, we only support setting TIDR for 'current' task.
  *