phy: atheros: Clarify the intention of ar8021_config

Debug register 5 contains TX_CLK DELAY at bit 8 and reserved values at
the other bit positions, just like the other PHYs in the family do.
Therefore, it is not necessary to hardcode the reserved values, but
instead simply follow the read-modify-write procedure from the common
function.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>