NTB: EPF: Fix error code in epf_ntb_bind()

Return an error code if pci_register_driver() fails.  Don't return
success.

Fixes: da51fd247424 ("NTB: EPF: support NTB transfer between PCI RC and EP connection")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Souptick Joarder (HPE) <jrdr.linux@gmail.com>
Signed-off-by: Jon Mason <jdmason@kudzu.us>