powerpc/ptrace: Fix build error in pkey_get()

The merge resolution in commit 25d8d4ee left ret no longer used,
leading to:

  arch/powerpc/kernel/ptrace/ptrace-view.c: In function ‘pkey_get’:
  arch/powerpc/kernel/ptrace/ptrace-view.c:473:6: error: unused variable ‘ret’
    473 |  int ret;

Fix it by removing ret.

Fixes: 25d8d4ee ("Merge tag 'powerpc-5.9-1' of git://git.kernel.org/pub/scm/linux/kernel/git/powerpc/linux"

)
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>