igbvf: add missing iounmap() on error in igbvf_probe()

Add the missing iounmap() before return from igbvf_probe()
in the error handling case.

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Tested-by: Sibai Li <Sibai.li@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>