MIPS: ath25: Check for kzalloc allocation failure

Currently there is no null check on a failed allocation of board_data,
and hence a null pointer dereference will occurr. Fix this by checking
for the out of memory null pointer.

Fixes: a7473717 ("MIPS: ath25: add board configuration detection")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: linux-mips@linux-mips.org
Cc: <stable@vger.kernel.org> # 3.19+
Patchwork: https://patchwork.linux-mips.org/patch/18657/


Signed-off-by: James Hogan <jhogan@kernel.org>