platform/x86: dell-wmi: check for kmalloc() errors

This allocation won't fail in the current kernel because it's small but
not checking for kmalloc() failures introduces static checker warnings
so let's fix it.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Mario Limonciello <mario.limonciello@dell.com>
Signed-off-by: Darren Hart (VMware) <dvhart@infradead.org>