x86/platform/intel-mid: Constify mid_pci_platform_pm

This struct never needs to be modified.  The size of pci-mid.o ELF
sections changes thusly:

  -.data          56
  +.data           0
  -.rodata        32
  +.rodata        88

Signed-off-by: Lukas Wunner <lukas@wunner.de>
Acked-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Bjorn Helgaas <bhelgaas@google.com>