p54: remove free_on_tx parameter

This patch simplifies the tx code a bit and will be necessary for
the upcoming stlc45xx<->p54 port.

In detail: we no longer have to tell all back-end drivers directly,
if we want to free a frame right after it was send to the firmware,
or if we do it in the library callback later.

Signed-off-by: Christian Lamparter <chunkeey@web.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>