igb: Avoid starting unnecessary workqueues

If ptp_clock_register() fails or CONFIG_PTP isn't enabled, avoid starting
PTP related workqueues.

In this way we can fix this:
 BUG: unable to handle page fault for address: ffffc9000440b6f8
 #PF: supervisor read access in kernel mode
 #PF: error_code(0x0000) - not-present page
 PGD 100000067 P4D 100000067 PUD 1001e0067 PMD 107dc5067 PTE 0
 Oops: 0000 [#1] PREEMPT SMP
 [...]
 Workqueue: events igb_ptp_overflow_check
 RIP: 0010:igb_rd32+0x1f/0x60
 [...]
 Call Trace:
  igb_ptp_read_82580+0x20/0x50
  timecounter_read+0x15/0x60
  igb_ptp_overflow_check+0x1a/0x50
  process_one_work+0x1cb/0x3c0
  worker_thread+0x53/0x3f0
  ? rescuer_thread+0x370/0x370
  kthread+0x142/0x160
  ? kthread_associate_blkcg+0xc0/0xc0
  ret_from_fork+0x1f/0x30

Fixes: 1f6e8178 ("igb: Prevent dropped Tx timestamps via work items and interrupts.")
Fixes: d339b133 ("igb: add PTP Hardware Clock code")
Signed-off-by: Alessio Igor Bogani <alessio.bogani@elettra.eu>
Tested-by: Arpana Arland <arpanax.arland@intel.com> (A Contingent worker at Intel)
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://lore.kernel.org/r/20230821171927.2203644-1-anthony.l.nguyen@intel.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>