gianfar: Cleanup/Fix gfar_probe and the hw init code

Factor out gfar_hw_init() to contain all the controller hw
initialization steps for a better control of register writes,
and to significantly simplify the tangled code from gfar_probe().
This results in code size and stack usage reduction (besides
code readability).

Fix memory leak on device removal, by freeing the rx_/tx_queue
structures.

Replace custom bit swapping function with a library one (bitrev8).

Move allocation of rx_/tx_queue struct arrays before the group
structure init, because in order to assign Rx/Tx queues
to groups we need to have the queues first.  This also allows
earlier bail out of gfar_probe(), in case the memory allocation
fails.

The flow control checks for maccfg1 were removed from gfar_probe(),
since flow control is disabled at probe time (priv->rx_/tx_pause_en
are 0). Redundant initializations (by 0) also removed.

Signed-off-by: Claudiu Manoil <claudiu.manoil@freescale.com>
Signed-off-by: David S. Miller <davem@davemloft.net>