ixgbe: Fix ixgbe_write_mbx error result

If ixgbe_write_mbx is called and no mbx->ops.write method
exists, no error code is returned. The corresponding read
function explicitly returns an error in such a case as do
other functions, so this appears to be a minor bug. Fix
it for consistency, and generate return values directly
to make things clearer.

Signed-off-by: Mark Rustad <mark.d.rustad@intel.com>
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>