powerpc: Fix build failure in sysdev/mpic.c for MPIC_WEIRD=y

Commit 446f6d06

 ("powerpc/mpic: Properly
set default triggers") breaks the mpc7447_hpc_defconfig as follows:

  CC      arch/powerpc/sysdev/mpic.o
arch/powerpc/sysdev/mpic.c: In function 'mpic_set_irq_type':
arch/powerpc/sysdev/mpic.c:886:9: error: case label does not reduce to an integer constant
arch/powerpc/sysdev/mpic.c:890:9: error: case label does not reduce to an integer constant
arch/powerpc/sysdev/mpic.c:894:9: error: case label does not reduce to an integer constant
arch/powerpc/sysdev/mpic.c:898:9: error: case label does not reduce to an integer constant

Looking at the cpp output (gcc 4.7.3), I see:

   case mpic->hw_set[MPIC_IDX_VECPRI_SENSE_EDGE] |
        mpic->hw_set[MPIC_IDX_VECPRI_POLARITY_POSITIVE]:

The pointer into an array appears because CONFIG_MPIC_WEIRD=y is set
for this platform, thus enabling the following:

  -------------------
  #ifdef CONFIG_MPIC_WEIRD
  static u32 mpic_infos[][MPIC_IDX_END] = {
        [0] = { /* Original OpenPIC compatible MPIC */

  [...]

  #define MPIC_INFO(name) mpic->hw_set[MPIC_IDX_##name]

  #else /* CONFIG_MPIC_WEIRD */

  #define MPIC_INFO(name) MPIC_##name

  #endif /* CONFIG_MPIC_WEIRD */
  -------------------

Here we convert the case section to if/else if, and also add
the equivalent of a default case to warn about unknown types.
Boot tested on sbc8548, build tested on all defconfigs.

Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>