net: sparx5: skip ptp deinit if init was skipped

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

net: sparx5: skip ptp deinit if init was skipped

sparx5_ptp_init() returns early on the base lan969x variants
because they do not have the SPX5_FEATURE_PTP flag. It also returns early
when no "ptp" interrupt is described. Unbinding the driver then causes a
NULL pointer dereference when cleaning up uninitialized tx_skbs queues:

Unable to handle kernel NULL pointer dereference at virtual address
0000000000000008
Call trace:
skb_queue_purge_reason+0x68/0x120 (P)
sparx5_ptp_deinit+0x64/0xc0
mchp_sparx5_remove+0x38/0x70
platform_remove+0x20/0x30
device_remove+0x4c/0x80

Before per-port tx_skbs were added, a NULL dereference would have happened
when calling ptp_clock_unregister() on the never-registered PTP clocks.

Fix by returning early in sparx5_ptp_deinit() if PTP is not used.