staging: wfx: check the return value of devm_kmalloc()

staging: wfx: check the return value of devm_kmalloc()

devm_kmalloc() returns a pointer to allocated memory on success, NULL
on failure. While there is a memory allocation of devm_kmalloc()
without proper check. It is better to check the return value of it to
prevent wrong memory access.
And I use the err label which is introduced by the previous patch to
handle the error.

Link: https://lore.kernel.org/r/tencent_24A24A3EFF61206ECCC4B94B1C5C1454E108@qq.com
Reviewed-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Jérôme Pouiller <jerome.pouiller@silabs.com>
Signed-off-by: Xiaoke Wang <xkernel.wang@foxmail.com>
Link: https://lore.kernel.org/r/tencent_D9887936F780A393C232DC48C9EC3F1D4405@qq.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>