mfd_get_cell returns a const, so change the ds1wm client to store
a const mfd cell. This silences type mismatch warnings.
Since we're guaranteed to have the mfd_cell, we can also simplify
the code a bit to get rid of a temporary variable and NULL check.
Signed-off-by: Andres Salomon <dilinger@queued.net>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>