igb: inline igb_maybe_stop_tx

igb_maybe_stop_tx() is extremely small and appears in several spots in
the tx hotpath. This change inlines the function for a possible
performance boost.

Signed-off-by: Nicholas Nunley <nicholasx.d.nunley@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>