net: phy: broadcom: use auxctl reading helper in BCM54612E code

Starting with commit 5b4e2900 ("net: phy: broadcom: add
bcm54xx_auxctl_read") we have a reading helper so use it and avoid code
duplication.
It also means we don't need MII_BCM54XX_AUXCTL_SHDWSEL_MISC define as
it's the same as MII_BCM54XX_AUXCTL_SHDWSEL_MISC just for reading needs
(same value shifted by 12 bits).

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>