p54: fix memory management

We have to be careful if multiple "control frames" are passed in a very short intervals to
the device's firmware. As p54_assign_address always put them into same memory location.
To guarantee that this won't happen anymore, we have to treat control frames like normal
data frames in the devices own memory management.

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