igb: Fix a memory leak in igb_probe

In error handling code of igb_probe, the memory adapter->shadow_vfta
allocated by kcalloc in igb_sw_init is not freed. So when register_netdev
or igb_init_i2c is failed, a memory leak will occur.
This patch adds kfree to fix it.

Signed-off-by: Jia-Ju Bai <baijiaju1990@163.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>