m68knommu: fix FEC platform device registration when driver is modular

The FEC (Fast Ethernet Crontroller) module on many ColdFire parts can
be compiled into the kernel, or as a module. Therefore the platform device
support for it is required whenever the driver is enabled - not just when
built into the kernel. Use IS_ENABLED(CONFIG_FEC) instead of a conditional
check on only the driver being built into the kernel.

Signed-off-by: Greg Ungerer <gerg@uclinux.org>
Acked-by: Geert Uytterhoeven <geert@linux-m68k.org>