IB/cma: Fix default RoCE type setting

The initial patch for changing the stack to use RoCEv2 GIDs by default
set the CMA_PREFERRED_ROCE_GID_TYPE to an incorrect value.  Instead of
an absolute value, we needed to set the right bit in a bitmask.  Correct
the default setting so we use RoCEv2 by default.

Fixes: 63a5f483 (IB/cma: Set default gid type to RoCEv2)
Signed-off-by: Doug Ledford <dledford@redhat.com>