net: fix mdio section mismatch warning

This fixes the following warning:

WARNING: drivers/net/phy/built-in.o(.devexit.text+0x70): Section mismatch in reference from the function .mdio_gpio_bus_destroy() to the function .devinit.text:.mdio_gpio_bus_deinit()
The function __devexit .mdio_gpio_bus_destroy() references
a function __devinit .mdio_gpio_bus_deinit().
This is often seen when error handling in the exit function
uses functionality in the init path.
The fix is often to remove the __devinit annotation of
.mdio_gpio_bus_deinit() so it may be used outside an init section.

Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>