net: dsa: lan9303: refactor lan9303_get_ethtool_stats

In lan9303_get_ethtool_stats: Get rid of 0x400 constant magic
by using new lan9303_read_switch_reg() inside loop.
Reduced scope of two variables.

Signed-off-by: Egil Hjelmeland <privat@egil-hjelmeland.no>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>