net: dsa: mv88e6xxx: fix handling of upper half of STATS_TYPE_PORT

Currently, the upper half of a 4-byte STATS_TYPE_PORT statistic ends
up in bits 47:32 of the return value, instead of bits 31:16 as they
should.

Fixes: 6e46e2d8 ("net: dsa: mv88e6xxx: Fix u64 statistics")
Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
Reviewed-by: Vivien Didelot <vivien.didelot@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>