common/verbs: fix usnic detection
    
The logic was mishandling the case of a newer kernel and an older
libusnic_verbs.  Simplify usnic_transport() to return constants in the
2 known cases (not a usNIC device and the TRANSPORT_USNIC_UDP case),
and call the magic probe in all other cases.

Reviewed-by: Dave Goodell <dgoodell@cisco.com>

cmr=v1.8.2:reviewer=ompi-rm1.8