powerpc: Drop unneeded cast in task_pt_regs()

There's no need to cast in task_pt_regs() as tsk->thread.regs should
already be a struct pt_regs. If someone's using task_pt_regs() on
something that's not a task but happens to have a thread.regs then
we'll deal with them later.
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200428123152.73566-1-mpe@ellerman.id.au

arch/powerpc/include/asm/processor.h

@@ -306,7 +306,7 @@ struct thread_struct {
 }
 #endif
 
-#define task_pt_regs(tsk)	((struct pt_regs *)(tsk)->thread.regs)
+#define task_pt_regs(tsk)	((tsk)->thread.regs)
 
 unsigned long get_wchan(struct task_struct *p);