igb: Fix potential invalid memory access in igb_init_module()

The pci_register_driver() can fail and when this happened, the dca_notifier
needs to be unregistered, otherwise the dca_notifier can be called when
igb fails to install, resulting to invalid memory access.

Fixes: bbd98fe4 ("igb: Fix DCA errors and do not use context index for 82576")
Signed-off-by: Yuan Can <yuancan@huawei.com>
Tested-by: Pucha Himasekhar Reddy <himasekharx.reddy.pucha@intel.com> (A Contingent worker at Intel)
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>