Octeontx2-af: Add proper checks for fwdata

Octeontx2-af: Add proper checks for fwdata

firmware populates MAC address, link modes (supported, advertised)
and EEPROM data in shared firmware structure which kernel access
via MAC block(CGX/RPM).

Accessing fwdata, on boards booted with out MAC block leading to
kernel panics.

Internal error: Oops: 0000000096000005 [#1]  SMP
[   10.460721] Modules linked in:
[   10.463779] CPU: 0 UID: 0 PID: 174 Comm: kworker/0:3 Not tainted 6.19.0-rc5-00154-g76ec646abdf7-dirty #3 PREEMPT
[   10.474045] Hardware name: Marvell OcteonTX CN98XX board (DT)
[   10.479793] Workqueue: events work_for_cpu_fn
[   10.484159] pstate: 80400009 (Nzcv daif +PAN -UAO -TCO -DIT -SSBS BTYPE=--)
[   10.491124] pc : rvu_sdp_init+0x18/0x114
[   10.495051] lr : rvu_probe+0xe58/0x1d18

Fixes: 997814491cee ("Octeontx2-af: Fetch MAC channel info from firmware")
Fixes: 5f21226b79fd ("Octeontx2-pf: ethtool: support multi advertise mode")
Signed-off-by: Hariprasad Kelam <hkelam@marvell.com>
Link: https://patch.msgid.link/20260121094819.2566786-1-hkelam@marvell.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>