ptp_clock_register fails on system with many PTP-capable NICs

Issue

• ptp_clock_register fails on system with many PTP-capable NICs
• On a system with more than 8 interfaces using the igb driver, the following messages are received (they can appear also for other drivers):

kernel: igb 0000:04:00.2: Intel(R) Gigabit Ethernet Network Connection
kernel: igb 0000:04:00.2: eth8: (PCIe:5.0Gb/s:Width x4) 00:19:99:c9:a2:7e
kernel: igb 0000:04:00.2: eth8: PBA No: D2745A-011
kernel: igb 0000:04:00.2: Using MSI-X interrupts. 8 rx queue(s), 8 tx queue(s)
kernel: igb 0000:04:00.3: PCI INT D -> GSI 46 (level, low) -> IRQ 46
kernel: igb 0000:04:00.3: added PHC on eth9
kernel: igb 0000:04:00.3: Intel(R) Gigabit Ethernet Network Connection
kernel: igb 0000:04:00.3: eth9: (PCIe:5.0Gb/s:Width x4) 00:19:99:c9:a2:7f
kernel: igb 0000:04:00.3: eth9: PBA No: D2745A-011
kernel: igb 0000:04:00.3: Using MSI-X interrupts. 8 rx queue(s), 8 tx queue(s)
kernel: igb 0000:07:00.0: PCI INT A -> GSI 16 (level, low) -> IRQ 16
kernel: igb 0000:07:00.0: ptp_clock_register failed
kernel: igb 0000:07:00.0: Intel(R) Gigabit Ethernet Network Connection
kernel: igb 0000:07:00.0: eth10: (PCIe:5.0Gb/s:Width x4) 00:19:99:c0:9e:97
kernel: igb 0000:07:00.0: eth10: PBA No: 333130-030
kernel: igb 0000:07:00.0: Using MSI-X interrupts. 8 rx queue(s), 8 tx queue(s)
kernel: igb 0000:07:00.1: PCI INT B -> GSI 17 (level, low) -> IRQ 17
kernel: igb 0000:07:00.1: ptp_clock_register failed