net: nixge: Fix error path for obtaining mac address

Fix issue where nixge_get_nvmem_address() returns a non-NULL
return value on a failed nvmem_cell_get() that causes an invalid
access when error value encoded in pointer is dereferenced.

Furthermore ensure that buffer allocated by nvmem_cell_read()
actually gets kfreed() if the function succeeds.

Fixes commit 492caffa

 ("net: ethernet: nixge: Add support for
National Instruments XGE netdev")
Reported-by: Alex Williams <alex.williams@ni.com>
Signed-off-by: Moritz Fischer <mdf@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>