net: phy: broadcom: add bcm_phy_modify_exp()

Add the convenience function to do a read-modify-write. This has the
additional benefit of saving one write to the selection register.

Signed-off-by: Michael Walle <michael@walle.cc>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>