IB/iser: Make reg_desc_get a per device routine

As for fmrs we will hold a single registration descriptor
as no need for multiple like in the frwr mode (descriptor
for each task). This change helps unifying the duplicate
registration code paths.

Signed-off-by: Sagi Grimberg <sagig@mellanox.com>
Signed-off-by: Adir Lev <adirl@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>