xprtrdma: Refactor frwr_is_supported

Refactor: Perform the "is supported" check in rpcrdma_ep_create()
instead of in rpcrdma_ia_open(). frwr_open() is where most of the
logic to query device attributes is already located.

The current code displays a redundant error message when the device
does not support FRWR. As an additional clean-up, this patch removes
the extra message.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>