net: pcs: xpcs: avoid reading STAT1 more than once

Avoid reading the STAT1 registers more than once while getting the PCS
state, as this register contains latching-low bits that are lost after
the first read.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>