RDMA/cma: Check that retry count values are in range

The retry_count and rnr_retry_count connection parameters are both
3-bit values.  Check that the values are in range and reduce if
they're not.

This fixes a problem reported by Doug Ledford <dledford@redhat.com>
that resulted in the userspace rping test (part of the librdmacm
samples) failing to run over Intel IB HCAs.

Signed-off-by: Sean Hefty <sean.hefty@intel.com>

[ Use min_t() to avoid warnings about type mismatch.  - Roland ]

Signed-off-by: Roland Dreier <roland@purestorage.com>