net: macb: free resources on failure path of at91ether_open()

DMA buffers were not freed on failure path of at91ether_open().
Along with changes for freeing the DMA buffers the enable/disable
interrupt instructions were moved to at91ether_start()/at91ether_stop()
functions and the operations on at91ether_stop() were done in
their reverse order (compared with how is done in at91ether_start()):
before this patch the operation order on interface open path
was as follows:
1/ alloc DMA buffers
2/ enable tx, rx
3/ enable interrupts
and the order on interface close path was as follows:
1/ disable tx, rx
2/ disable interrupts
3/ free dma buffers.

Fixes: 7897b071 ("net: macb: convert to phylink")
Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>