w1: Silence ds1wm warnings related to mfd_get_cell changes

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>