net: aquantia: temperature retrieval fix

Chip temperature is a two byte word, colocated internally with cable
length data. We do all readouts from HW memory by dwords, thus
we should clear extra high bytes, otherwise temperature output
gets weird as soon as we attach a cable to the NIC.

Fixes: 8f894011 ("net: aquantia: add infrastructure to readout chip temperature")
Tested-by: Holger Hoffstätte <holger@applied-asynchrony.com>
Signed-off-by: Igor Russkikh <igor.russkikh@aquantia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>