w1: Use device platform_data to retrieve ds1wm platform bits

With the addition of the platform device mfd_cell pointer, we can now
cleanly pass the sub device drivers platform data pointers through the
regular device platform_data one, and get rid of mfd_get_data().

Cc: Matt Reimer <mreimer@vpop.net>
Cc: Philipp Zabel <philipp.zabel@gmail.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>