macvlan: cleanup rx statistics

We have very similar code for rx statistics in
two places in the macvlan driver, with a third
one being added in the next patch.

Consolidate them into one function to improve
overall readability of the driver.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>