phy/micrel: move flag handling to function for common use

The flag MICREL_PHY_50MHZ_CLK is not of exclusive use of KSZ8051
model. At least KSZ8021 and KSZ8031 models also use it.
This patch moves the handling of this and future flags to a
separate function so that the different PHY models can call it on
their init function, if needed.

Signed-off-by: Hector Palacios <hector.palacios@digi.com>
Signed-off-by: David S. Miller <davem@davemloft.net>