sfp: fix oops with ethtool -m

If a network interface is created prior to the SFP socket being
available, ethtool can request module information.  This unfortunately
leads to an oops:

Unable to handle kernel NULL pointer dereference at virtual address 00000008
pgd = (ptrval)
[00000008] *pgd=7c400831, *pte=00000000, *ppte=00000000
Internal error: Oops: 17 [#1] SMP ARM
Modules linked in:
CPU: 0 PID: 1480 Comm: ethtool Not tainted 4.19.0-rc3 #138
Hardware name: Broadcom Northstar Plus SoC
PC is at sfp_get_module_info+0x8/0x10
LR is at dev_ethtool+0x218c/0x2afc

Fix this by not filling in the network device's SFP bus pointer until
SFP is fully bound, thereby avoiding the core calling into the SFP bus
code.

Fixes: ce0aa27f

 ("sfp: add sfp-bus to bridge between network devices and sfp cages")
Reported-by: Florian Fainelli <f.fainelli@gmail.com>
Tested-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>