IB: Set class_dev->dev in core for nice device symlink

All RDMA drivers except ehca set class_dev->dev to their dma_device
value (ehca leaves this unset).  dma_device is the only value that
makes any sense, so move this assignment to core/sysfs.c.  This reduce
the duplicated code in the rest of the drivers and gives ehca a nice
/sys/class/infiniband/ehcaX/device symlink.

Signed-off-by: Joachim Fenkes <fenkes@de.ibm.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>