ixgbevf: ixgbevf_write/read_posted_mbx should use IXGBE_ERR_MBX to initialize ret_val

Now ixgbevf_write/read_posted_mbx use -IXGBE_ERR_MBX as the initiative
return value, but it's incorrect, cause in ixgbevf_vlan_rx_add_vid(),
it use err == IXGBE_ERR_MBX, the err returned from mac.ops.set_vfta,
and in ixgbevf_set_vfta_vf, it return from write/read_posted. so we
should initialize err with IXGBE_ERR_MBX, instead of -IXGBE_ERR_MBX.

With this fix, the other functions that called it also can work well,
cause they only care about if err is 0 or not.

Signed-off-by: Xin Long <lucien.xin@gmail.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>