igc: fix page fault when thunderbolt is unplugged

After unplug thunderbolt dock with i225, pciehp interrupt is triggered,
remove call will read/write mmio address which is already disconnected,
then cause page fault and make system hang.

Check PCI state to remove device safely.

Trace:
BUG: unable to handle page fault for address: 000000000000b604
Oops: 0000 [#1] SMP NOPTI
RIP: 0010:igc_rd32+0x1c/0x90 [igc]
Call Trace:
igc_ptp_suspend+0x6c/0xa0 [igc]
igc_ptp_stop+0x12/0x50 [igc]
igc_remove+0x7f/0x1c0 [igc]
pci_device_remove+0x3e/0xb0
__device_release_driver+0x181/0x240

Fixes: 13b5b7fd ("igc: Add support for Tx/Rx rings")
Fixes: b03c49cd

 ("igc: Save PTP time before a reset")
Signed-off-by: Aaron Ma <aaron.ma@canonical.com>
Tested-by: Dvora Fuxbrumer <dvorax.fuxbrumer@linux.intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>