Mark switch cases where we are expecting to fall through.

Fixes errors such as below, seen with mpc85xx_defconfig:

  arch/powerpc/kernel/align.c: In function 'emulate_spe':
  arch/powerpc/kernel/align.c:178:8: error: this statement may fall through
    ret |= __get_user_inatomic(temp.v[3], p++);
        ^~
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20190730141917.21817-1-mpe@ellerman.id.au