We are currently doing auxiliary control register reads with the shadow
register value 0b111 (0x7) which incidentally is also the selector value
that should be present in bits [2:0]. Fix this by using the appropriate
selector mask which is defined (MII_BCM54XX_AUXCTL_SHDWSEL_MASK).

This does not have a functional impact yet because we always access the
MII_BCM54XX_AUXCTL_SHDWSEL_MISC (0x7) register in the current code.
This might change at some point though.

Fixes: 5b4e2900

 ("net: phy: broadcom: add bcm54xx_auxctl_read")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>