net/mlx5e: Do not use ptp structure for tx ts stats when not initialized

The ptp channel instance is only initialized when ptp traffic is first
processed by the driver. This means that there is a window in between when
port timestamping is enabled and ptp traffic is sent where the ptp channel
instance is not initialized. Accessing statistics during this window will
lead to an access violation (NULL + member offset). Check the validity of
the instance before attempting to query statistics.

  BUG: unable to handle page fault for address: 0000000000003524
  #PF: supervisor read access in kernel mode
  #PF: error_code(0x0000) - not-present page
  PGD 109dfc067 P4D 109dfc067 PUD 1064ef067 PMD 0
  Oops: 0000 [#1] SMP
  CPU: 0 PID: 420 Comm: ethtool Not tainted 6.9.0-rc2-rrameshbabu+ #245
  Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS Arch Linux 1.16.3-1-1 04/01/204
  RIP: 0010:mlx5e_stats_ts_get+0x4c/0x130
  <snip>
  Call Trace:
   <TASK>
   ? show_regs+0x60/0x70
   ? __die+0x24/0x70
   ? page_fault_oops+0x15f/0x430
   ? do_user_addr_fault+0x2c9/0x5c0
   ? exc_page_fault+0x63/0x110
   ? asm_exc_page_fault+0x27/0x30
   ? mlx5e_stats_ts_get+0x4c/0x130
   ? mlx5e_stats_ts_get+0x20/0x130
   mlx5e_get_ts_stats+0x15/0x20
  <snip>

Fixes: 3579032c

 ("net/mlx5e: Implement ethtool hardware timestamping statistics")
Signed-off-by: Rahul Rameshbabu <rrameshbabu@nvidia.com>
Signed-off-by: Tariq Toukan <tariqt@nvidia.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>