ethdev: remove useless parameter in callback process

ethdev: remove useless parameter in callback process

The pointer to the user parameter of the callback registration is
automatically pass to the callback function.
There is no point to allow changing this user parameter by a caller.
That's why this parameter is always set to NULL by PMDs and set only
in ethdev layer before calling the callback function.

The history is that the user parameter was initially used
by the callback implementation to pass some information
between the application and the driver:
	c1ceaf3ad056 ("ethdev: add an argument to internal callback function")
Then a new parameter has been added to leave the user parameter
to its standard usage of context given at registration:
	d6af1a13d7a1 ("ethdev: add return values to callback process API")

The NULL parameter in the internal callback processing function
is now removed. It makes clear that the callback parameter is user
managed and opaque from a DPDK point of view.

Signed-off-by: Thomas Monjalon <thomas@monjalon.net>
Reviewed-by: Ferruh Yigit <ferruh.yigit@intel.com>