i40e: Check and set the PF driver state first in i40e_ndo_set_vf_mac

The PF driver state flag __I40E_VIRTCHNL_OP_PENDING needs to be
checked and set at the beginning of i40e_ndo_set_vf_mac. Otherwise,
if there are error conditions before it, the flag will be cleared
unexpectedly by this function to cause potential race conditions.
Hence move the check to the top of this function.

Signed-off-by: Lihong Yang <lihong.yang@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>