IB/ehca: Activate scaling code by default

Change ehca's Kconfig to activates scaling code as default.  After
several measurements we saw that this feature prevents dropped packets
(UD) in stress situation. Thus, enabling it helps to improve ehca's
bandwidth through IPoIB.

Signed-off-by: Hoang-Nam Nguyen <hnguyen@de.ibm.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>