ibmvnic: Correct return code checking for ibmvnic_init during probe

The update to ibmvnic_init to allow an EAGAIN return code broke
the calling of ibmvnic_init from ibmvnic_probe. The code now
will return from this point in the probe routine if anything
other than EAGAIN is returned. The check should be to see if rc
is non-zero and not equal to EAGAIN.

Without this fix, the vNIC driver can return 0 (success) from
its probe routine due to ibmvnic_init returning zero, but before
completing the probe process and registering with the netdev layer.

Fixes: 6a2fb0e9 (ibmvnic: driver initialization for kdump/kexec)
Signed-off-by: Nathan Fontenot <nfont@linux.vnet.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>