ixgbe: fix Kconfig when driver is not a module

The new ability added to the driver to use mii_bus to handle MII related
ioctls is causing compile issues when the driver is compiled into the
kernel (i.e. not a module).

The problem was in selecting MDIO_DEVICE instead of the preferred PHYLIB
Kconfig option.  The reason being that MDIO_DEVICE had a dependency on
PHYLIB and would be compiled as a module when PHYLIB was a module, no
matter whether ixgbe was compiled into the kernel.

CC: Dave Jones <davej@codemonkey.org.uk>
CC: Steve Douthit <stephend@silicom-usa.com>
CC: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Reviewed-by: Stephen Douthit <stephend@silicom-usa.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>