IB/ipath: Remove ib_sg_dma_address() and ib_sg_dma_len() overloads

The removal of these methods is compensated for by code changes to
.map_sg to insure that the vanilla sg_dma_address() and sg_dma_len()
will do the same thing as the equivalent former ib_sg_dma_address()
and ib_sg_dma_len() calls into the drivers.

The introduction of this patch required that the struct
ipath_dma_mapping_ops be converted to a C99 initializer.

Suggested-by: Bart Van Assche <bvanassche@acm.org>
Reviewed-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Mike Marciniszyn <mike.marciniszyn@intel.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>