If an error is encountered in ntb_device_setup, it is possible that the
spci_cmd isn't populated.  Writes to the offset can result in a NULL
pointer dereference.  This issue is easily encountered by running in
NTB-RP mode, as it currently is not supported and will generate an
error.  To get around this issue, return if an error is encountered
prior to attempting to write to the spci_cmd offset.
Signed-off-by: NJon Mason <jon.mason@intel.com>