RDMA/usnic: Instantiate data structures once

Data structures should not be defined in a header file. Hence move
the min_transport_spec[] definition from a header file to a .c file.

Signed-off-by: Bart Van Assche <bart.vanassche@sandisk.com>
Cc: Christian Benvenuti <benve@cisco.com>
Cc: Dave Goodell <dgoodell@cisco.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>