atmel: use netstats in net_device structure

Use net_device_stats from net_device structure instead of local.
Kill atmel_get_stats function, because by default it is used
identical internal_stats function from net/core/dev.c
No need to memset stats to 0, because they are allocated by kzalloc.

P.S. Someone should cleanup init_atmel_card function from unneeded
initializations to 0/NULL. Out of scope for this patch.

Signed-off-by: Paulius Zaleckas <paulius.zaleckas@teltonika.lt>
Signed-off-by: John W. Linville <linville@tuxdriver.com>