staging/rdma: Kconfig change STAGING_RDMA to be tristate.

STAGING_RDMA was failing to build when INFINIBAND was set to 'm' and
STAGING_RDMA was set to 'y'.

Making this a tristate properly inherits the 'm' from the INFINIBAND setting.

Reviewed-by: Dalessandro, Dennis <dennis.dalessandro@intel.com>
Reviewed-by: John, Jubin <jubin.john@intel.com>
Reviewed-by: Mike Marciniszyn <mike.marciniszyn@intel.com>
Signed-off-by: Ira Weiny <ira.weiny@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>