MIPS: lantiq: check the return value of kzalloc()

[ Upstream commit 34123208 ]

kzalloc() is a memory allocation function which can return NULL when
some internal memory errors happen. So it is better to check the
return value of it to prevent potential wrong memory access or
memory leak.

Signed-off-by: Xiaoke Wang <xkernel.wang@foxmail.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Signed-off-by: Sasha Levin <sashal@kernel.org>