mxc_fec: avoid free() calls to already freed pointers.

Sometimes, inside NetLoop, eth_halt() is called before eth_init() has
been called. This is harmless except for free() calls to pointers
which have not been allocated yet.

This patch initializes those pointers to NULL and allocates them only
the first time. This way we can get rid of free calls in halt callback.

This has been tested in i.MX27 Litekit board and eldk-4.2 toolchains.

Signed-off-by: Javier Martin <javier.martin@vista-silicon.com>
Signed-off-by: Ben Warren <biggerbadderben@gmail.com>