sh/tlb: Fix PGTABLE_LEVELS > 2

Geert reported that his SH7722-based Migo-R board failed to boot after
commit:

  c5b27a88 ("sh/tlb: Convert SH to generic mmu_gather")

That commit fell victim to copying the wrong pattern --
__pmd_free_tlb() used to be implemented with pmd_free().

Fixes: c5b27a88

 ("sh/tlb: Convert SH to generic mmu_gather")
Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Tested-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Rich Felker <dalias@libc.org>