net: dsa: bcm_sf2: fix 64-bits register reads

Reading 64-bits register was not working because we inverted the steps
between reading the lower 32-bits of the register and reading the upper
32-bits. Swapping these operations is how the HW guarantees that 64-bits
reads are latched correctly. We only have a handful of 64-bits registers
for now, mostly MIB counters, so the imapct is low.

Fixes: 246d7f77 ("net: dsa: add Broadcom SF2 switch driver")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>