ixbge: remove TX lock and redo TX accounting.

This ports Herbert Xu's "maybe_stop_tx" code and removes the tx_lock
which is not needed.

Signed-off-by: Ayyappan Veeraiyan <ayyappan.veeraiyan@intel.com>
Signed-off-by: Auke Kok <auke-jan.h.kok@intel.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>