MFD: twl6040: Remove global pointer for platform_device

There is no need to keep global pointer for the platform
device, since it is only used for dev_* prints, and the
device pointer available within the twl6040 structure.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Acked-by: Samuel Ortiz <sameo@linux.intel.com>
Acked-by: Liam Girdwood <lrg@ti.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>