net/mlx5: Fix bridge mode operations when there are no VFs

In the Linux kernel, the following vulnerability has been resolved:

net/mlx5: Fix bridge mode operations when there are no VFs

Currently, trying to set the bridge mode attribute when numvfs=0 leads to a

crash:

bridge link set dev eth2 hwmode vepa

[ 168.967392] BUG: kernel NULL pointer dereference, address: 0000000000000030

[...]

[ 168.969989] RIP: 0010:mlx5_add_flow_rules+0x1f/0x300 [mlx5_core]

[...]

[ 168.976037] Call Trace:

[ 168.976188] <TASK>

[ 168.978620] _mlx5_eswitch_set_vepa_locked+0x113/0x230 [mlx5_core]

[ 168.979074] mlx5_eswitch_set_vepa+0x7f/0xa0 [mlx5_core]

[ 168.979471] rtnl_bridge_setlink+0xe9/0x1f0

[ 168.979714] rtnetlink_rcv_msg+0x159/0x400

[ 168.980451] netlink_rcv_skb+0x54/0x100

[ 168.980675] netlink_unicast+0x241/0x360

[ 168.980918] netlink_sendmsg+0x1f6/0x430

[ 168.981162] ____sys_sendmsg+0x3bb/0x3f0

[ 168.982155] ___sys_sendmsg+0x88/0xd0

[ 168.985036] __sys_sendmsg+0x59/0xa0

[ 168.985477] do_syscall_64+0x79/0x150

[ 168.987273] entry_SYSCALL_64_after_hwframe+0x76/0x7e

[ 168.987773] RIP: 0033:0x7f8f7950f917

(esw->fdb_table.legacy.vepa_fdb is null)

The bridge mode is only relevant when there are multiple functions per

port. Therefore, prevent setting and getting this setting when there are no

VFs.

Note that after this change, there are no settings to change on the PF

interface using `bridge link` when there are no VFs, so the interface no

longer appears in the `bridge link` output.