Due to a typo we would always be using the MIB counter width of the
first element of the counter array instead of the current element, and
we would always be accessing the register statistics with a 64-bits
read, while some could be 32-bits. This got unnoticed in testing with
MDIO and SRAB which tolerate doing this, but testing with the SPI bus
revealed bogus values being returned. Fix this by using the proper
iterator here.

Fixes: 967dd82f ("net: dsa: b53: Add support for Broadcom RoboSwitch")
Reported-by: Jonas Gorski <jogo@openwrt.org>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>