octeon_ep: Validate the VF ID

octeon_ep: Validate the VF ID

Add a helper to validate the VF ID and use it in the VF ndo ops to
prevent accessing out-of-range entries.

Without this check, users can run commands such as:

 # ip link show dev enp135s0
 2: enp135s0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq state UP mode DEFAULT group default qlen 1000
    link/ether 00:00:00:01:01:00 brd ff:ff:ff:ff:ff:ff
    vf 0     link/ether 00:00:00:00:00:00 brd ff:ff:ff:ff:ff:ff, spoof checking on, link-state enable, trust off
    vf 1     link/ether 00:00:00:00:00:00 brd ff:ff:ff:ff:ff:ff, spoof checking on, link-state enable, trust off
 # ip link set dev enp135s0 vf 4 mac 00:00:00:00:00:14
 # echo $?
 0

even though VF 4 does not exist, which results in silent success instead
of returning an error.

Fixes: 8a241ef9b9b8 ("octeon_ep: add ndo ops for VFs in PF driver")
Signed-off-by: Kamal Heib <kheib@redhat.com>
Reviewed-by: Michal Swiatkowski <michal.swiatkowski@linux.intel.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20250911223610.1803144-1-kheib@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>