net/mlx5: Fix command stats access after free

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

net/mlx5: Fix command stats access after free

Command may fail while driver is reloading and can't accept FW commands
till command interface is reinitialized. Such command failure is being
logged to command stats. This results in NULL pointer access as command
stats structure is being freed and reallocated during mlx5 devlink
reload (see kernel log below).

Fix it by making command stats statically allocated on driver probe.

Kernel log:
[ 2394.808802] BUG: unable to handle kernel paging request at 000000000002a9c0
[ 2394.810610] PGD 0 P4D 0
[ 2394.811811] Oops: 0002 [#1] SMP NOPTI
...
[ 2394.815482] RIP: 0010:native_queued_spin_lock_slowpath+0x183/0x1d0
...
[ 2394.829505] Call Trace:
[ 2394.830667] _raw_spin_lock_irq+0x23/0x26
[ 2394.831858] cmd_status_err+0x55/0x110 [mlx5_core]
[ 2394.833020] mlx5_access_reg+0xe7/0x150 [mlx5_core]
[ 2394.834175] mlx5_query_port_ptys+0x78/0xa0 [mlx5_core]
[ 2394.835337] mlx5e_ethtool_get_link_ksettings+0x74/0x590 [mlx5_core]
[ 2394.836454] ? kmem_cache_alloc_trace+0x140/0x1c0
[ 2394.837562] __rh_call_get_link_ksettings+0x33/0x100
[ 2394.838663] ? __rtnl_unlock+0x25/0x50
[ 2394.839755] __ethtool_get_link_ksettings+0x72/0x150
[ 2394.840862] duplex_show+0x6e/0xc0
[ 2394.841963] dev_attr_show+0x1c/0x40
[ 2394.843048] sysfs_kf_seq_show+0x9b/0x100
[ 2394.844123] seq_read+0x153/0x410
[ 2394.845187] vfs_read+0x91/0x140
[ 2394.846226] ksys_read+0x4f/0xb0
[ 2394.847234] do_syscall_64+0x5b/0x1a0
[ 2394.848228] entry_SYSCALL_64_after_hwframe+0x65/0xca