net: phy: broadcom: drop duplicated define for RGMII SKEW delay

We had two defines for the same bit (both were used with the
MII_BCM54XX_AUXCTL_SHDWSEL_MISC register).

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>